Behavioral psychology basically means that you can create behaviors in people, or get them to stop behaviors, by training them much the way you'd train a puppy. Pavlov did an experiment with dogs where he made them salivate, a completely biological response, at the sound of a bell. If you give a dog a treat while ringing a bell, you are basically connection the two responses. In essence, our brains can be trained!
Behaviorism is one field of study in psychology interested in understanding what drives human behavior.
One of the central ideas of behaviorism is the idea of feedback. Feedback can be defined loosely as stimulus following behavior. For instance, if a student studies hard and gets an "A" on an exam. The "A" is feedback for the behavior (the studying).
Feedback can be both positive and negative, either reinforcing behavior or creating an aversion to certain behavior.
This field of study is far more complex than a thumbnail sketch can express, but this is one approach to studying behavior.