Operant conditioning, also called instrumental conditioning, is a method for modifying behavior (an operant) which utilizes contingencies between a discriminative stimulus, an operant response, and a reinforcer to change the probability of a response occurring again in that situation. This method is based on Skinner's three-term contingency and it differs from the method of Pavlovian conditioning.
An everyday illustration of operant conditioning involves training your dog to "shake" on command. Using the operant conditioning technique of shaping, you speak the command to "shake" (the discriminative stimulus) and then wait until your dog moves one of his forepaws a bit (operant response). Following this behavior, you give your dog a tasty treat (positive reinforcer). After demanding ever closer approximations to shaking your hand, your dog finally comes to perform the desired response to the verbal command "shake."