Behaviourism is an approach to learning developed at the turn of the twentieth century which focuses on the scientific description of behaviour, often generalising from animal experiments (cf Pavlov, Thorndike) to humans (cf Watson).
It considers the subject or learner as a tabula rasa and avoids reference to internal mental states. The learner is influenced by his or her environment (stimulus) to which he or she reacts (response) and in turn receives a reaction (reinforcement; Skinner).
Learning is thus a history of the subject's interactions with the environment and the onus for learning is on the teacher, who manipulates the environment in order to stimulate appropriate reactions in learners. Later behaviourists proposed that learners also affect their environment (cf Bandura, modelling).