Trichomes are little glassy/crystally looking beads that begin to appear on the buds (and may fall on the leaves if it is a potent strain). They contain THC and other cannabinoids. They start off clear and after reaching maturity they begin to turn cloudy, then amber or golden. When some begin to turn amber is generally the best time to harvest, but it is up to individual taste. Harvesting with only a few amber tric's will result in an up heady high, while mostly amber will give you a stoned body high. Harvesting in between will give you a high somewhere between the heady cerebral and stoned body highs, depending on how long you wait to harvest. Don't overshoot harvesting by waiting until all the tric's are brown!!! They begin to lose potency once most of them turn darker in color.

Pic is some clear trichomes (from Eirik at Overgrow.com/norcan.org - used with permission).