Honey is a food product produced by bees from plant nectar. It is a common sweetener, and has many nutritional and antibiotic properties. It is used in beer as either a fermentable sugar or a priming agent. Honey is the primary (or only) fermentable in mead.
Honey as a fermentable sugar
Honey is about 95% fermentable sugar, and will be slightly more fermentable than an equivalent wieght of malt extract. It does not contribute significantly to the body of the beer, so replacing an amount of extract with honey will lighten the beer accordingly.
Brewing with honey
Honey contributes flavors to the beer if used as 5-10% or more of the fermentable sugar in the wort. If honey contributes more than 30% of the fermentable sugars to a beer, it is braggot, not beer.
Boiling destroys or alters many of the flavoring compounds in honey, so if it is added to the boil, the flavors will be subtle. Honey can be pasteurized instead of boiled, retaining more of the flavor profile in the finished beer. Raw honey should never be added to a fermentation due to high risk of bacterial infection and loss of the batch.
Priming with honey
Honey can be used for priming sugar in bottle conditioning. A typical 5 gallon batch will use 6-10 ounces of honey, or 1/2 cup to 7/8 cup. Honey used for priming should be boiled or pasteurized.