Compound exercises work several different muscles at once, allowing you to work most of the major muscles in the body in just a few moves. Isolation exercises work just one or two muscles.
Muscles generally come in opposing pairs (eg. biceps to bend the elbow, triceps to straighten it), so even with compound exercises, you will need to do 2 for each muscle group.
Unident's list of compound exercises is a good one, although I would add deadlifts to that list.