Improvisational theatre, often called improv or impro, is a form of theater where most or all of what is performed is created at the moment it is performed. In its purest form, the dialogue, the action, the story and the characters are created collaboratively by the players as the improvisation unfolds in present time, without use of an already prepared, written script.
Improvisational theatre exists in performance as a range of styles of improvisational comedy as well as some non-comedic theatrical performances. It is sometimes used in film and television, both to develop characters and scripts and occasionally as part of the final product.
Improv Encyclopedia Fantastic website with nearly 500 Theatresports™ games.
Learn Improv Useful website on all things improv with exercise and games etc.
Wikipedia Entry on Improvisation.
Wikipedia Entry on Improvisational Comedy.
Wikipedia Handy list of several improvisational theatre companies in Australia, Canada, United States and Europe.