What the hell is Austrian theory of the business cycle?

I read some opinions/posts from Bryan Caplan's EconLog. I always wonder where is such thing "the Austrian theory of the business cycle" comes from. Because this kind of theory looks very stupid, you do not even want to pay any attention on it. Caplan is a smart man, I believe. But why he talks about this "Austrian theory of the business cycle", sometimes if not always?

I don't know who is working on this theory, or who believes it. But I know Milton Friedman said he did not believe any kind business cycle theory any bit.

References:
Hal Varian on Modern Capital
Why I Am Not an Austrian Economist
A Conversation with Milton Friedman (this is the Milton Friedman's last interview which is organized by the Dallas Fed, about one year before he died.)