The main assumption of the behaviourist perspective is that all behaviour is learned and shaped by the environment.
The behaviourist perspective also argues that in order for psychology to be scientific it should focus on observable behaviour which can be objectively measured rather than on things like cognitive processes which can only be inferred.
Two important learning theories proposed by the behaviourist perspective are classical conditioning (Pavlov) and operant conditioning (Skinner). Classical conditioning explains how we learn behaviours through association and operant conditioning explains how the consequences of behaviours (reinforcers) shape behaviour.
A main strength of the behaviourist perspective has been the development of useful applications. Behaviourism offers very practical ways of changing behaviour from, for example, therapies through to advertising.
Perhaps the main problem with the behaviourist approach occurs because by not focusing on cognitive processes it is only giving a partial explanation of human experience. A further problem with the behavioural perspective is that many of the practical uses of the approach when used as a way of changing behaviour do tend to be short lived. That is, they do change behaviour but often only for a limited time.
In relation to qualitative research, behaviourism is an outside - in approach toward human behaviour. It is only interested in identifying or manipulating the stimulus - and measuring the response or change in behaviour. It has relevance to ethnomethodological studies but it does not deal with human subjective meaning making it very limiting for many qualitative endeavours.