Improv Comedy
Improvisational theatre (also known as improv or impro) is a form of theatre in which the actors use improvisational acting techniques to perform spontaneously. Actors typically use audience suggestions to guide the performance as they create dialogue, setting, and plot extemporaneously. Improvisational theatre performances tend to be comedic, although some forms, including Playback Theatre and Theatre of the Oppressed, are not necessarily intended to be comedic.
Many improvisational actors also work as scripted actors, and "improv" techniques are often taught in standard acting classes. The basic skills of listening, clarity, confidence, and performing instinctively and spontaneously are considered important skills for actors to develop.
Source: http://en.wikipedia.org/wiki/Improvisational_theatre