Mead is a fermented drink made from honey. It is thought to be the oldest fermented drink made by man. The Phrase "Nectar of the Gods" refers to Mead. Mead is also considered to be the source of the phrase "honeymoon" - a woman's dowry was meant to contain enough mead to supply the new husband for a month (perhaps by that time it'd be too late to back out?).
If you are interested in brewing your own mead, read this page through, then go to the mead brewing page.
Broken down to its most basic, mead is simply honey, water and yeast mixed and left for a period of time - as determined by your own personal taste. If you add anything else to the above mix, it becomes something slightly different, and can roughly be split into two main categories:
If you add spices or herbs to mead, it becomes metheglin/metheglyn. This word is related to the current-day word for medicine - adding Welsh llyn (liquor) to the latin: Medicus (medicine). This relation is due to the fact that many medical herbs/spices could be easily kept for long periods in the mead - which acts as a great preservative... not to mention its benefits in making it go down easy.
When you add fruits or berries etc to the mix, it becomes a melomel. There are *many* different, very successful, melomels around; and these old favourties often have their own, special name - strawberry (or any other berry) is oft-used, though I don't know a name for it, grapes (pyment), and apples (cyser) are also pretty common.
Sure, there are always a few things that fall through the holes too. Hydromel seems to be like that - which is a mead that has a higher percentage of water. I believe this was often used more as a medicinal, but can't be sure. Hippocras was apparently a hydromel/melomel.
Sack is also fairly well-known as a type of mead and simply refers to a very thick, syrupy wine - easy enough to do when you add lots and lots of honey (usually about 12kg to the imperial gallon).