Behaviorism is an approach to study behavior based only on what can be directly seen. Behaviorists focus on relationships between stimuli and responses.

Unseen qualities such as states of mind (any condition which is significantly different from a normal waking state e.g. as a result of fear or anxiety) were not used in this type of study even though we know that the mind plays a part in all advanced animals' behaviors. Behaviorism states that behavior can be studied without knowing what the physiology of an event is, and without using theories such as that of the mind. By definition, all behavior can be observed.

Behaviorism also relied on another idea, that all human behavior was learned. Behaviorists believed that behavior could be explained by classical or operant conditioning. This is learning as a result of influences from past experiences.  However, behaviorists denied the importance of inherited behaviors, instincts (inherent inclination of a living organism), or inherited tendency to behave. They did not believe, or ignored, the idea of heredity(passing of traits to offspring from parents), that something can come from a person's genes. This was the idea of the blank slate, that babies are born with a clean, empty mind.  Humans, when born, are thought not to have mental experience or knowledge, and that everything is learned after they grow. The blank slate premise is opposed by modern evolutionary psychology.

Major contributors, scientists to the field of behaviorism include C. Lloyd Morgan, Ivan Pavlov, Edward Thorndike, John B. Watson and B.F. Skinner.

Pavlov researched classical conditioning through the use of dogs and their natural ability to salivate, produce water in their mouths. Thorndike and Watson rejected looking at one's own conscious thoughts and feelings ("Introspection"). They wanted to restrict psychology to experimental methods. Skinner's research leant mainly on behavior shaping using positive reinforcement (rewards rather than punishments).

Today, ideas from behaviorism are used in cognitive-behavioral therapy. Cognitive-behavioral therapy can help people deal with anxieties and phobias, as well as with certain forms of addiction.

As a scientific theory, behaviorism has largely been replaced by cognitive psychology.

Conditioning 
The act of conditioning is when a desired behavior is made through training. This is done by pairing stimuli with a specific behavior. Some behaviors are natural reflexes which  people (and animals) are born with. Infants are born with inherited reflexes that help them eat, communicate, and survive. These reflexes are unconditioned, they are not taught to the baby.

Classical conditioning 

Classical conditioning (also known as Pavlovian conditioning) is when a conditioned stimulus causes an unconditioned response. This explains how people gained new responses to different stimuli.

Another example of an unconditioned response is when wind is blown in a person's eyes and they blink automatically to prevent dust or something from getting into them. This is a reflex that is innate.

Fear conditioning is when a previously neutral stimulus is used to elicit fear. One main example is the Little Albert experiment by Watson and Rayner. The researchers tested infants' emotional reactions. They found that Little Albert would react to a loud noise and then consequently conditioned that noise to elicit fear when he saw a white rat. This came to be known as 'conditioned emotional response'. After a period of time, Little Albert would cry when he saw a white rat or anything small and white, even his stuffed animal.

Operant conditioning 

Operant conditioning is also known as instrumental conditioning. It was studied by Thorndike and Skinner.

Related pages 
Blank slate
Empiricism
Human nature
Nature versus nurture
Psychological nativism

References

Psychology