Honey was the original sweetener, known and consumed by our ancestors since time immemorial. It is suitable for immediate consumption in its unprocessed state, unlike any other major sweetener, requiring only the skill or foolhardiness to steal it from the bees. In its pure state honey will not ferment, but fermentation can occur by accident or be deliberately induced.
Pure Honey
In its pure state, honey is a highly concentrated sugar containing very little water. This makes it one of the very few foods that is naturally resistant to spoilage, because yeasts and other microorganisms require a degree of free water in order to thrive and reproduce. This characteristic was well known in ancient times, and the use of honey to dress wounds is mentioned in the Bible. When stored carefully, honey will darken and crystallize but will not ferment.
Unintentional Fermentation
Honey ferments and spoils under certain circumstances. If the honey is harvested too early when its moisture content is high, or if it isn't stored airtight and absorbs moisture from the atmosphere, it can ferment. When this happens the yeast spores in the honey wake from their dormant state and begin digesting the sugars, processing them into alcohol. The honey will foam visibly and develop unpleasantly sour flavors. At this point the honey is considered unfit for human consumption and most beekeepers feed it back to the bees.
Deliberate Fermentation
Fermentation, of course, is how humans produce alcoholic beverages -- and fermented honey may be one of the oldest of all. To ferment honey deliberately, it is dissolved into water with yeast added. Over time the wine will ferment like wine or beer, producing a tart honey wine called mead. Mead was deeply ingrained in ancient Celtic and Nordic cultures, and our term "honeymoon" refers to a month when the newlyweds were largely left alone to drink mead and get to know each other.
Types of Mead
Mead is produced in a great variety of styles, reflecting its diverse ancestry. There is an entire vocabulary relating to the different varieties of mead. A mead with apples added during fermentation is a cyser, one with grapes is a pyment and one with any other fruit added is a melomel. A mead with spices added is metheglin, and one with rose petals is a rhodomel. A quickly fermented ale-like mead is a short mead, while one intended for long-term storage is a great mead. The mead equivalent of dessert wine is called a sack mead.