Compression is an oft-misunderstood and sometimes over-used effect that enjoys (?) a wealth of online forum punditry. Sonic Sense has done an exemplary job here in plainly illustrating the rudiments of compression while cutting the crap. This video begins by demonstrating exactly what the basic compressor controls do, and then follows up with audible examples of the compressor being applied during tacking AND in a full mix, so you can very clearly hear the effect applied in the context of a full song. Demos include vocals, snare, and bass.
Thanks to Sonic Sense for not only clearing up some of the mysteries of compression, but also for choosing the ADL 700 as the right tool for the job.