In snowboarding (or skiing), Black Diamond runs represent the most difficult runs. They are usually steep with complicated terrain. They can be a lot of fun, but if there are lots of moguls, its sort of a pain on a snowboard. The runs are usually marked with a black diamond next to the name - harder runs will have two or three diamonds (double black diamond or triple black diamond). If you're a newbie at snowboarding, beware of these... I've slid down a few on my ass, and not willingly.

I'm not sure how the classification works but it seems pretty arbitrary. I've seen moderate runs at sugar bowl that are more difficult than black diamond runs at snow summit.