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Seeing as there is many, many times when this question is answered repeatedly, I decided to make a thread to explain it, and I will add it to the "stickies."
First off, it's important to know that custom ROMs, unlocked bootloaders, and root are three very different things. While they tend to be thrown around loosely and some people do not know the difference, you CAN technically have any one of these things, alone, or a combination.
Bootloaders.
Bootloader, or boot.img, is, as it sounds, the first thing your phone finds when you initialize the boot. In the past, they were left fairly open to modification and the like, but in the past year or two more and more companies are locking phones down tighter. The development community generally finds ways around locked bootloaders, but there is no guarantees. An unlocked bootloader makes obtaining root and custom ROMs MUCH easier. There are ways to get around it (mostly something called kexec), but most are buggy and somewhat inconvenient, and very complicated so I won't explain them here. You can do that research if it pertains to you.
Here's a list of Android manufacturers and their stances on bootloaders. These are the ones I'm familiar with, feel free to let me know of others.
Samsung - Leaves it alone, rarely locks them. They did lock the Verizon Galaxy S3 and Verizon Note 2 by Verizon's request, and both were cracked (or for the GS3, an unlocked one was leaked). Generally, as far as I'm aware, Samsung has not encrypted any other bootloader on any other carrier.
HTC - Originally left them fairly wide open, then began locking them down. Upon user backlash, they created an unlock tool which allows unlocking of most old and almost all new devices. Some have been blocked by carriers (ATT One X, Verizon Rezound) but I believe these restrictions were bypassed and HTCdev has been successful for almost all of their new devices. HTC bootloaders use something called S-on and S-off. The S stands for security. Devices come S-on and developers work towards S-off. HTCdev does not grant S-off, but comes pretty close. HTC is not especially great at locking them tight, so S-off is very often obtained regardless. Generally, custom ROMs are very possible on HTC devices.
Motorola - Motorola is essentially a curse word to many devs at this point. They have a policy of locking up almost all their devices. Following user backlash, they said they would try to offer unlockable devices, and have slightly kept that promise. In Europe most Motorola devices now are unlockable, but in the US, very few are, and none on Verizon (the majority of their devices) are unlockable. They're also very good at locking them. I'm only aware of two Moto phones in recent years that have been cracked, and I believe they were because of a leaked passcode. Kexec is mostly used on Motorola devices because of this policy.
LG - Unsure, can someone post LG's stance so i can add it here?
Sony - Sony is very open about it and even encourages unlocking of bootloaders and I believe even honors warranties. Google has even accepted one Sony phone into the AOSP because of their very open approach.
Asus - Mixed bag. Original Transformer was unlockable but I believe recent ones have been locked. It's my understanding they do have an unlock tool available if you're willing to void your warranty.
Nexus - Since Nexus devices may come from these manufacturers but have a very different policy, I'll touch on them. Every Nexus thus far can be easily unlocked in about five minutes with very little risk involved. They come directly from Google and Google supports that.
Root
Having root access means being able to read and write to the lower level of the system. Android phones do NOT come pre-rooted, but almost all mainstream devices have been rooted. It generally is much easier to obtain root (especially on Motorolas) than unlock the bootloader. The specifics aren't important, but gaining root modifies the permissions in your phone. In some ways it increases security, but puts you in more control. Gaining root includes deploying an app called either Superuser or SuperSU (both do the same thing, SuperSU is gaining more popularity these days). Every time root access is requested by an app, SuperSU/Superuser will prompt you to either grant or deny that app access. Having root opens up a massive amount of new apps available through Google Play Store, including backup apps, system tools, and almost anything else under the sun. While customization without root is very possible, root allows for much deeper customization and theming. While you do not need to have an unlocked bootloader to have root, if you do have an unlocked bootloader, root is obtained VERY easily by flashing a simple zip file through recovery. OTA updates almost always break root, so especially if you've got a Motorola, research new updates before you update, in order to make sure root can be regained.
Custom ROMs
The thing root and unlocked bootloaders work towards. It's very in depth, but I'll try to explain it in layman's terms. Custom ROMs are versions of Android modified by anyone who chooses, and distributed to people to run on their phones. Often they are debloated (carrier junk removed), have performance modifications or battery life changes, and may come themed. These are often used to be running the latest version of Android before the manufacturer or carrier pushes the update out. As they are typically done as a hobby, they will usually have more bugs than a standard ROM, especially when new. There are two different types of major custom ROMs. AOSP or stock. Stock is built on top of the original ROM or one from a similar phone. They will be TouchWiz (Samsung) Sense (HTC) or Blur (Motorola) based ROMs and look similar to what came on your phone. On the other hand, AOSP (Android Open Source Project) ROMs are versions based on the vanilla android Google produces, that exists on Nexus devices. it has no Touchwiz, Sense, or Blur. It's a preference. These may have more bugs as they're significantly different from the manufacturer's software. Popular AOSP distrobutions include CyanogenMod, AOKP, and many others. Almost all custom ROMs come pre-rooted.
Kernels are the lower level of the ROM. I'm not an expert on kernels to be honest, but these are what allow overclocking. They tend to vary by device and some may be buggier or have bad battery life, so you'll have to test these yourself. Be sure to flash the right kernel, however. AOSP kernels and TouchWiz/Sense kernels are not interchangeable usually. For the most part, kernels cannot be changed without an unlocked bootloader, but custom ROMs sometimes can be changed a bit (though it's much harder for developers!).
Recovery
Recovery mode is on every phone I'm aware of. It's the mechanism through which updates are installed. However, stock recoveries, even on Nexus devices, do not allow flashing of unsigned files. That's where custom recoveries come in. Popular recoveries include ClockworkMod and TWRP. They are accessed by pressing a key combination whilst turning your phone on, and allow nandroid backups (full system image backups), restores, and allow unsigned images to be flashed. Custom recovery, to my knowledge, can be flashed on any device with root, and root can be gained by any device with custom recovery. to find which comes first you'll have to research your device. Recovery is the console through which all custom ROMs are flashed. Even most locked bootloaders allow custom recoveries.
If you have any suggestions, please go ahead and post them up! Hope this helps clear these things up..
First off, it's important to know that custom ROMs, unlocked bootloaders, and root are three very different things. While they tend to be thrown around loosely and some people do not know the difference, you CAN technically have any one of these things, alone, or a combination.
Bootloaders.
Bootloader, or boot.img, is, as it sounds, the first thing your phone finds when you initialize the boot. In the past, they were left fairly open to modification and the like, but in the past year or two more and more companies are locking phones down tighter. The development community generally finds ways around locked bootloaders, but there is no guarantees. An unlocked bootloader makes obtaining root and custom ROMs MUCH easier. There are ways to get around it (mostly something called kexec), but most are buggy and somewhat inconvenient, and very complicated so I won't explain them here. You can do that research if it pertains to you.
Here's a list of Android manufacturers and their stances on bootloaders. These are the ones I'm familiar with, feel free to let me know of others.
Samsung - Leaves it alone, rarely locks them. They did lock the Verizon Galaxy S3 and Verizon Note 2 by Verizon's request, and both were cracked (or for the GS3, an unlocked one was leaked). Generally, as far as I'm aware, Samsung has not encrypted any other bootloader on any other carrier.
HTC - Originally left them fairly wide open, then began locking them down. Upon user backlash, they created an unlock tool which allows unlocking of most old and almost all new devices. Some have been blocked by carriers (ATT One X, Verizon Rezound) but I believe these restrictions were bypassed and HTCdev has been successful for almost all of their new devices. HTC bootloaders use something called S-on and S-off. The S stands for security. Devices come S-on and developers work towards S-off. HTCdev does not grant S-off, but comes pretty close. HTC is not especially great at locking them tight, so S-off is very often obtained regardless. Generally, custom ROMs are very possible on HTC devices.
Motorola - Motorola is essentially a curse word to many devs at this point. They have a policy of locking up almost all their devices. Following user backlash, they said they would try to offer unlockable devices, and have slightly kept that promise. In Europe most Motorola devices now are unlockable, but in the US, very few are, and none on Verizon (the majority of their devices) are unlockable. They're also very good at locking them. I'm only aware of two Moto phones in recent years that have been cracked, and I believe they were because of a leaked passcode. Kexec is mostly used on Motorola devices because of this policy.
LG - Unsure, can someone post LG's stance so i can add it here?
Sony - Sony is very open about it and even encourages unlocking of bootloaders and I believe even honors warranties. Google has even accepted one Sony phone into the AOSP because of their very open approach.
Asus - Mixed bag. Original Transformer was unlockable but I believe recent ones have been locked. It's my understanding they do have an unlock tool available if you're willing to void your warranty.
Nexus - Since Nexus devices may come from these manufacturers but have a very different policy, I'll touch on them. Every Nexus thus far can be easily unlocked in about five minutes with very little risk involved. They come directly from Google and Google supports that.
Root
Having root access means being able to read and write to the lower level of the system. Android phones do NOT come pre-rooted, but almost all mainstream devices have been rooted. It generally is much easier to obtain root (especially on Motorolas) than unlock the bootloader. The specifics aren't important, but gaining root modifies the permissions in your phone. In some ways it increases security, but puts you in more control. Gaining root includes deploying an app called either Superuser or SuperSU (both do the same thing, SuperSU is gaining more popularity these days). Every time root access is requested by an app, SuperSU/Superuser will prompt you to either grant or deny that app access. Having root opens up a massive amount of new apps available through Google Play Store, including backup apps, system tools, and almost anything else under the sun. While customization without root is very possible, root allows for much deeper customization and theming. While you do not need to have an unlocked bootloader to have root, if you do have an unlocked bootloader, root is obtained VERY easily by flashing a simple zip file through recovery. OTA updates almost always break root, so especially if you've got a Motorola, research new updates before you update, in order to make sure root can be regained.
Custom ROMs
The thing root and unlocked bootloaders work towards. It's very in depth, but I'll try to explain it in layman's terms. Custom ROMs are versions of Android modified by anyone who chooses, and distributed to people to run on their phones. Often they are debloated (carrier junk removed), have performance modifications or battery life changes, and may come themed. These are often used to be running the latest version of Android before the manufacturer or carrier pushes the update out. As they are typically done as a hobby, they will usually have more bugs than a standard ROM, especially when new. There are two different types of major custom ROMs. AOSP or stock. Stock is built on top of the original ROM or one from a similar phone. They will be TouchWiz (Samsung) Sense (HTC) or Blur (Motorola) based ROMs and look similar to what came on your phone. On the other hand, AOSP (Android Open Source Project) ROMs are versions based on the vanilla android Google produces, that exists on Nexus devices. it has no Touchwiz, Sense, or Blur. It's a preference. These may have more bugs as they're significantly different from the manufacturer's software. Popular AOSP distrobutions include CyanogenMod, AOKP, and many others. Almost all custom ROMs come pre-rooted.
Kernels are the lower level of the ROM. I'm not an expert on kernels to be honest, but these are what allow overclocking. They tend to vary by device and some may be buggier or have bad battery life, so you'll have to test these yourself. Be sure to flash the right kernel, however. AOSP kernels and TouchWiz/Sense kernels are not interchangeable usually. For the most part, kernels cannot be changed without an unlocked bootloader, but custom ROMs sometimes can be changed a bit (though it's much harder for developers!).
Recovery
Recovery mode is on every phone I'm aware of. It's the mechanism through which updates are installed. However, stock recoveries, even on Nexus devices, do not allow flashing of unsigned files. That's where custom recoveries come in. Popular recoveries include ClockworkMod and TWRP. They are accessed by pressing a key combination whilst turning your phone on, and allow nandroid backups (full system image backups), restores, and allow unsigned images to be flashed. Custom recovery, to my knowledge, can be flashed on any device with root, and root can be gained by any device with custom recovery. to find which comes first you'll have to research your device. Recovery is the console through which all custom ROMs are flashed. Even most locked bootloaders allow custom recoveries.
If you have any suggestions, please go ahead and post them up! Hope this helps clear these things up..
