Compression is probably one of the most misunderstood aspects of recording. Essentially compressors tighten the dynamics of an instrument, vocal or mix.
The way they do this is by bringing down the loudest parts or 'peaks' and bringing up the quietest parts.
They allow the dynamics of the music to exist in a more controlled dynamic parameter. If done correctly the result is a more full and powerful sounding mix.
The end result is controlled dynamics that can be reproduced much more efficiently on a broader set of musical system such as radio, boom boxes as well as high quality stereos.
Its important to keep in mind that recording is an artificial way of reproducing sound. Because of this, audio reproduction has its limitations. Compressors help music systems reproduce sound more efficiently.
There is an art to using compressors correctly. It is not simply just a matter of turning a few dials to get the desired results. Compressors work with dynamics.
Musical vitality is dependent of reproducing its dynamics in a natural way. Understanding how to hear and work with these dynamics is the goal of the professional recording engineer.
Effects like reverb and chorus are fairly obvious in the way they work. It takes a bit more skill to properly restructure the dynamics of music so they remain musical. This is something that takes time, patience or training.