A compressor controls the level of the signal going into it by reducing the volume of the loud parts, and increasing the volume of the soft parts to make the signal overall more consistent. Most compressors have 2 variables, compression, which is how much to compress the signals, and release, which is how long to hold the compression for, the release can be used to increase the sustain of the notes. Compression is normally used on vocals, bass, and drums, as well as other instruments as needed. It can be used while recording, or in the final mix down. Most people prefer to run everything "dry" (no compression or other extra effects) into the mixer, use the sends to go out to the effects, and then record, adding the final effects during the mastering.
I remember when some of the people who are now bitching about people who ask questions before doing a search were exactly the same people who did the same!
Think of these redundent questions as a way to reaffirm what you know. Trust me, some of you need the reminding....
There is a link in that article that goes to an article I wrote about compression too. But Shailat's article has audio examples so you can hear what different compressor settings will do. My article explains more the basics of compression, and what each knob does.
I hear what you are saying Ed, but I was just trying to give cdfain a little nudge in the right direction. Same as I got when I first posted a question here without searching many moons ago. Wasn't trying to be mean spirited in any way, so don't take it that way cdfain.