Compound exercises are those that involve movement at more than one joint and utilize more than one muscle group at the same time. For example, when performing the bench press, the shoulder joint and elbow joint are involved in pressing the weight.
During isolation exercise, on the other hand, one muscle group is used and movement only occurs at one joint. An example of a single joint chest exercise would be a flat dumbbell fly. In this exercise, the shoulder joint is the only joint where movement occurs.
Isolation exercises are often recommended to correct muscle imbalance or weakness that often occurs after an injury. Isolating a specific muscle is sometimes necessary to get it to activate and increase its strength.
In almost all cases, it is easier to use heavier weight with compound exercises when compared to isolation exercises. Also, compound moves tend to recruit more muscle fibers and burn more calories than single joint exercises. Compound exercises most simulate real life activities and improve joint stability and muscle balance across a joint which helps improve coordination, reaction time and balance.