Behaviorism is a learning theory that focuses on objectively observable behaviors: what can be seen, heard, measured, weighed; since mental processes, such as thinking and feeling, are so difficult to register, they relied on objective behavior, that can be described scientifically.
The russian researcher Ivan Pavlov was main precursor with his investigations on classical conditioning, which is one of the ways of learning.
Pavlov was investigating the digestion of the dogs and observed that they salivated when food was presented, and later began salivating in the presence of the lab aid who fed them; salivating in presence of the food was an unconditioned response, but in the presence of the lab aid was a learned, conditioned response, that became associated with the food.
American John B. Watson was the first psychologist who called himself behaviorist and centered his research on observable behaviors; American psychologist B.F. Skinner conducted research on operant conditioning, which is another way of learning, more complex than the classical conditioning, more frequent, more human.
In this form of learning, behaviors are maintained by its consequences, basically reinforcement, punishment
But human mind is too complex and it was somehow necessary to deal with many processes that are not directly observable, opened, but rather covered (feelings, emotions, thinking); behaviorist therapy works very well with kids and today is used with adults, too, in combination with other techniques.