Cricket question: What's a "test match?"

[VGGAlan]

On the BBC World Service, the sports (or "sport" ;^) ) broadcasts generally include info about cricket "test matches." At first, I assumed a test match was like a scrimmage or a pre-season game: fun to watch, but it doesn't really matter in the grand scheme of things. But some of these test matches sound like they are pretty serious -- almost like playoff games.

So I'm turning to the teeming millions. What exactly is a "test match?"

-alan