Flash floods are a phenomenon that occurs when an area is hit with a large amount of water in a short amount of time. They are usually produced by rain clouds, though they can also result from things like dam breaks and landslides.
In areas within the direct watershed of mountains, you might find flash floods running tens and even hundreds of miles from the high elevations where the rains first fell. These kinds of flash floods can come hours after the feeding storms pass and can be dangerous to unwary people in its path. Imagine waking up in the middle of the night in your desert campsite as your tent is swept away by a dark flood.
Self proclaimed "flash flood chaser" David Rankin filmed this awesome video showing how a flash flood looks as it finds its way through a dried out riverbed.
David's YouTube page has a great collection of other flash flood videos that are worth the time to watch.
Check out these articles about flooding here on MNN: