This article is from the Fitness FAQ, by Jeff Gleixner (firstname.lastname@example.org) with numerous contributions by others.
A warm-up is an activity that gets your muscles "warmed up".
Usually a 5 minute bike ride at a steady comfortable pace, or a
brisk 5 minute walk, is enough. Most people like to go until they
"break a sweat". A cool-down is just the opposite. You want to
gradually slow the body down. This is usually done by biking/walking
at a fairly fast pace and gradually slowing down over a few minutes.
Stretching also makes up a part of the cool down.