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What's the difference between compound and isolation movements?
Strictly speaking, compound involves multiple joints. Isolation involves one joint. A biceps curl, for example, is an isolation movement because, when done correctly (and I say this because many people allow their upper arm to move and thereby involve the shoulder joint as well) the only joint that has movement is the elbow joint. There are actually multiple muscles involved, but a single joint.
The squat, on the other hand, involves static contract with your shoulder and elbow joints, then movement about your hip, knee, and ankle joints, etc, so definitely compound. A Bench press involves your shoulder and your elbow joint - compound. A triceps kickback? Only the elbow, isolation. A leg extension? Only the knee ... isolation.
Think "joints."