Mead is a fermented Honey drink. Nectar of the Gods as it was once called. Is one of the oldest forms of fermented drink recorded. There are several variations in the mead as there are with wine. There are of course more than I have listed here but these are the major ones home brewers may be interested in.
Traditional Mead: Honey, water and yeast fermentations. Deep gold honey color to almost no coloring. Dry to extremely sweet, there is also a wide variety in taste due to the honey variations.
Cyser is a blend of honey and Apples/Apple juice and as with all mead varies by honey types as well as the apple used.
Melomel is a broad term meaning honey and any fruit. Several have been broken into specific names like Cyser (Apple), Pyment (Grapes), or Morat (mulberries). The term Melomel is used more for the fruits not specifically listed above. More like a Strawberry Melomel or Blueberry Melomel.
Metheglin uses a traditional mead with the addition of spices or Herbs. You see this most with ginger, cinnamon, vanilla, sage, oranges or peels.
Braggot started as mead using hops but has evolved to honey and malt but may or may not have hops.
Two other terms you should know is Sack mead and Show mead. Sack mead is a very sweet mead usually refered to as mead that has been sweetened after fermentation. Show mead is refered to as a mead that is a honey water base only. Traditional mead is what most competitions will refer to this.