Absurdist theater is a phrase coined by Martin Esslin, a theather critic who grouped together several playwrights who wrote in a similar fashion. Absurdist drama is a direct result of World War 2, and is about the way people felt after it. There are three types of Absurdism: Fatalism, Existentialism, and Hilarious. Fatalism is the type of theater Beckett wrote (Waiting for Godot, End Game), where man is trapped in life, even basic communication is cut off. Existentialism is that if there is no point, at least you can rationalize an existance, No Exit is an example of this. Hilarious is the type Ionesco wrote (The Lesson), based off the Neitzsche prinicpal that man invented laughter to keep sanity. The entire point of Absurdism is that there is no point to life on the surface, so there has to be some reason to live. It's an attempt to answer to the fundemental questions, why are we here? etc.
One might argue that Samuel Beckett, the author of Waiting fo Godot, is the best absurdist playwright, even though crictics hated it until a group of prison inmates did a production of it.