Compound exercises involve multiple muscles and joints. For instance,
the military press and the squat compound exercises.  The military press 
involves the elbow and shoulder joints and the front shoulder muscle 
and triceps muscle.  Therefore, it's a compound exercise.  

The squat involves the joints of the knees, hips, back and the muscles 
in the upper thighs, buttocks and back.  It also is a compound exercise.

On the other hand, exercises such as the Barbell curl and Heel raises
are isolation exercises.  That means they only work one joint and one 
muscle.  The barbell curl works the elbow joint and the biceps muscle. 

And the heel raises involve the joint of the ankle and the calf muscle of 
the lower leg.  Most of the exercises in this routine are compound exercises 
although there are a few isolation exercises as well.

Compound exercises are best for building over all body strength and 
conditioning.  This is usually what most people want to accomplish.