Grappling is a generic term to describe combat in very close quarters or on the ground, with or without strikes. The goal of a grappler is to make an opponent submit, or give up. This is usually achieved by means of joint locks, compressions, and chokes. If the opponent doesn't give up once a submission move has "sunk in," they will quickly become incapacitated, either because they are unconscious or have a broken limb.
All moves when done with correct form should rely mainly on leverage, and not brute strength-- although strength does help. One often-used technique in grappling is tiring the opponent by making them work as hard as possible, without one's self becoming too tired. This method makes it easier to bring an opponent into submission.