Compound exercises work several muscle groups at once, and include movement around two or more joints. For example, in the leg press movement occurs around the hip, knee and ankle joints. This exercise is primarily used to develop the quadriceps, but it also involves the hamstrings, glutes and calves.
Compound exercises are generally similar to the ways that people naturally push, pull and lift objects, whereas isolation exercises often feel a little unnatural. Compound exercises generally involve dumbbells and barbells (free weights), involving more muscles to stabilize the body and joints as well as move the weight.
Compound exercises build the basic strength that is needed to perform everyday pushing, pulling and lifting activities. Those who seek to increase their strength, muscle mass, or performance in sports would focus mostly on compound exercises.