'The Man' explains 'The Theory'
To see a Quicktime video of B.F Skinner explaining his own theory - click on the picture above.

Operant Conditioning - in a nutshell
Operant Conditioning is a technique applied to increase the probability of emitted responses through application of reinforcing consequences. ie: An unconditioned hungry rat is placed into the 'Skinner Box'. The rat is conditioned to associate the sound of the food magazine with the delivery of food pellets. These stimuli are paired until the rat immediately goes to the food tray after hearing the click of the magazine. (The lever may or may not have been present during the first phase of the procedure, but any response to the lever was not reinforced.)The rat's behavior now needs to be SHAPED because pressing a lever (or writing a letter)is not a natural thing that the rat would do. Any behavior that leads to the pressing of the lever is reinforced. Such as: moving into the half of the box where the lever is, raising up on it's hind legs, or putting a paw up.
Once the rat has pushed the lever a few times, it is conditioned to press the lever by pairing only this behavior with the sound of the magazine (which has already been paired with the reinforcement).<

Link to the B.F Skinner Foundation, an indepth explanation of Operant Conditioning.