Operant conditioning is the use of consequences to modify the occurrence and form of behavior. Operant conditioning is distinguished from classical conditioning (also called respondent conditioning, or Pavlovian conditioning) in that operant conditioning deals with the modification of “voluntary behavior” or operant behavior. Operant behavior “operates” on the environment and is maintained by its consequences, while classical conditioning deals with the conditioning of respondent behaviors which are elicited by antecedent conditions. Behaviors conditioned via a classical conditioning procedure are not maintained by consequences. The main dependent variable is the rate of response that is developed over a period of time. New operant responses can be further developed and shaped by reinforcing close approximations of the desired response.
Source: http://en.wikipedia.org/wiki/Operant_conditioning