An operational amplifier has many purposes, most of them being defined by how you place the resistors in relation to the inputs and output terminals of the opamp.
A single DIP-8 (8 pin package) opamp has five different, useful pins if you draw the circuit diagram representation. First, you have the power pins that allow a positive and a negative voltage (these voltages are typically equal in magnitude but different in sign). Second, you have the input pins. There's a positive and a negative input. Based on how you place the resistors relative to those pins and the output pin, you can adjust the gain of the circuit. By gain, I mean output voltage divided by input voltage.
Next, I'll draw up two fundamental opamp circuits for you to feast your eyes on. Input signal is a 1V amplitude 1kHz signal.
First up is the inverting amplifier. R1 is 1k, connected between signal source and negative terminal. R2 is 10k, connected between the output and the negative terminal. The gain of this is -(R2/R1) = -10. Notice that there is a 180º phase shift (positive input becomes negative output).
Circuit:
Output and Input as a function of time:
The other fundamental circuit is the non-inverting amplifier. In this case, the input is placed at the positive terminal but the negative terminal still does the biasing and adjust the gain. As the name implies, the output is not inverted and stays in phase with the original signal. In this case, the gain of the amplifier is (R2/R1)+1 = 11.
Circuit:
Output and Input as a function of time:
All simulations done with TINA, a tool provided by Texas Instruments utilizing the OPAy132 model.
Typically, adjusting voltage gain means you are making the speaker or headphone you're driving louder or quieter.