I don't know what exact ally it is called, but your computer saves address to files on you Hard Drive
When you "delete" a file windows only deletes the address, not the actual data.
The computer no longer knows the data is there, so it does not appear. However, The data is still in perfect condition until another file is placed on top of it.
Programs can scan your data and find the files again quite easily.
This is what a security format is for. (usually writes all 0s, then random 1 and 0, then all 0 again)
The government actually had a hard drive that had 32 random passes of data put on it and restored the original information (though, that would have taken a LONG time...)
is nice for ACTUALLY deleting things. You can set it to perform 7 passes on the files that it deletes.
It will also fix registry errors...quite a handy FREE program!