Compound exercises work multiple muscles across more then one joint. Bench press, dips, and squats are examples of compound exercises. Isolation exercises work fewer muscles across a single joint. Dumbbell flyes, pec deck, and leg extensions are examples of isolation exercises.
Some people believe that in order to target a certain muscle you should use more isolation exercises and thus develop that muscle more efficiently. However, this is not the best approach. Isolation exercises that work the muscles across a single joint are not the most effective way to build muscle. Compound exercises are far more effective in building muscle because they incorporate the use of several muscle groups, allow for a heavier training load, and work more muscle mass in the same amount of time.
Let’s look at the squat. It is a compound exercise. It works the quadriceps, hamstrings, gluteus, lower back, abdominals, hips, and abductors. Squats allow for heavy weights to be lifted. This makes the squat an incredibly effective muscle building exercise. Compared to an isolation exercise such as the leg extension, which only focuses on the quadriceps. From this simple example you can clearly see how much more productive compound exercises are for building muscle.