A slang term used to refer to the crystals (sometimes described as fine red hairs) that one can shake or scrape off high-quality marijuana bud. The term probably evolved from the more general Indian word 'kef'.
You can use a razor blade to scrape keef off freshly cured buds, or you can use a keef box. Once collected, the crystals may be smoked like an extremely potent joint. Some people sprinkle keef on a packed bowl before smoking to increase the potency of their average bud. Many people collect their keef in glass vials, which makes for an attractive display.
Marijuana dealers will sometimes (some say often) collect the shake from a bunch of excellent kind bud and then sell the bud and the keef seperately. However, they are able to do this without reducing the price they charge for the bud, thereby increasing their profit significantly.