There are two different kinds of yeast nutrient available. They are yeast hulls (also known as `ghosts') and di-ammonium phosphate (usually just called `yeast nutrient'--this is the easiest to find). Some people say that the yeast hulls make for better meads more quickly, since you don't have to wait for the chemical taste from the phosphates to wane. My opinion is that both are usable, but the yeast-hulls leave you more margin for error. If you put in too many hulls, you'll just have a larger layer of sediment to deal with. If you put in too much phosphate, you'll have a chemical taste that'll take a while to fade.
In older times, egg white was also used as a yeast nutrient, but given the problems with salmonella in eggs nowadays, I'd recommend against using this method. That said, to use 'em, you basically just whip up an egg white until you've got something resembling meringue, break that up into little bits, and toss it into the must. If more than roughly a third of your fermentable sugars are coming from fruit or barley malt, you won't need to worry about yeast nutrients, since these ingredients contain the things yeast will need.