Body language is a broad term for several forms of communication between two or more people using body movements instead of sounds or other media. In turn, it is one category of paralanguage, which describes all forms of human communication that are not language. Paralanguage including body language has been extensively studied in social psychology.
The subject can be divided into at least two major groups: voluntary and involuntary body language.
The first kind, perhaps less commonly discussed because it seems unproblematic, refers to movement, gestures and poses intentionally made by the subject: moving hands, mimating actions, and generally making movements with full or partial intention of making them. It can apply to many types of soundless communication, for example, formalized gestures in matches.
The second kind is often what one means when talking about body language: involuntary movements that may give observers a clue about what one is really thinking or feeling. The ability of spotting the movements is itself subconscious, at least for non-trained people.
The relation of body language to animal communication has often been discussed. Human paralanguage may represent a continuation of forms of communication that our non-linguistic ancestors already used, or it may be that it has been changed by co-existing with language. Some species of animals are especially adept at detecting human body language, both voluntary and involuntary: this is the basis of the Clever Hans effect (a source of artefact in comparative psychology), and was also the reason for trying to teach the chimpanzee Washoe American Sign Language rather than speech - and perhaps the reason why the Washoe project was more successful than some previous efforts to teach apes human languages.