Behavior therapy is a therapy that is conducted by a therapist that has been trained for this. This therapy is based on the notion that certain behavior (like an addiction) starts by rewarding this behavior, and that it is reduced by punishment. This rewarding and punishing shouldn't be seen in the literal sense of the words. A reward can be that you win something from a slot machine, that you get more friends by using drugs, or that you feel better by smoking. In other words, it is something positive resulting from certain behavior. Punishment can be that you have a terrible hangover and feel bad after drinking, that you lose friends because you use drugs, or that you have debts because you gamble.
The therapy uses rewards and punishments to achieve more good behavior (be motivated, not using) and to reduce negative behavior (relapse).
Rewards can consist of a night to the movies, a new sweater, going out for dinner, a day off, etc., when during a determined period of time you have behaved well. Punishments can consist of prohibition to receive visitors, no extras, not being allowed to go home for the weekend, or in worse cases informing employers or a family member (with the permission of the addict).