A wee bit more technical this!

What's it all about?

In 1938, Burrhus Skinner, a behaviourist psychologist, published what was almost certainly the most influential work on the behaviour of animals of the twentieth century: "The Behaviour of Organisms". Before him, Edward Tolman had made extensive studies of the behaviour of rats exploring mazes, concentrating on their response to varied and various stimuli. Likewise, Edward Thorndike had attempted to develop an objective experimental method for the mechanical problem-solving ability of cats and dogs.
Skinner's theory is based on the idea that learning is a function of change in overt
behaviour. Changes in behaviour are the result of an individual's response to events, or stimuli, that occur in the environment. When a particular Stimulus-Response (S-R) pattern is reinforced, by reward, the individual is then conditioned to respond.
Reinforcement is the key element in Skinner's S-R theory. A reinforcer is anything that strengthens the desired response. In the case of my llamas it is a little bit of grain to eat.The theory also covers negative reinforcers - any stimulus that results in the increased frequency of a response when it is withdrawn, as distinct from adversive stimuli (punishment) which result in reduced responses. A great deal of attention was devoted to schedules of reinforcement e.g. interval, ratio,etc. and their effects on establishing and maintaining behaviour patterns.
Where classical conditioning illustrates S-R learning, operant conditioning is often viewed as R-S learning since it is the consequence that follows the response that influences whether the response is likely or unlikely to occur again. It is through operant conditioning that voluntary responses are learned.
It is the stimulus that follows a voluntary response (i.e., the response's consequence) that changes the probability of whether the response is likely or unlikely to occur again.