What does a compressor do? Why are pedals so noisy? Where in the signal chain should you put it?

00:00 beginning
00:35 "I can't hear it doing anything"
03:24 "This is ruining my dynamics!"
05:12 "The sound is too 'loose' or floppy sounding"
07:50 "This compressor is too noisy!" (Applies to distortions and overdrives as well)
09:05 "Always put compression before drive pedals" or, "Always put compression after drive pedals"