Compression is the second most important part of a mix. There are three main reasons that I like to use compression. They are good for controlling volume and dynamic range, fattening up tracks, and creating energy in a track. I have created a presentation to give a basic run down of compression and what different parts of a compressor do. Follow the link below to check it out.
Compression is something that can make or break your entire mix. It is absolutely crucial to have a plan when using compression. Trial and error is definitely involved, but you need to at least know why you are compressing your track to get the desired result.
As always, just press record; you never know what may come out!