A d100 — also called a d%, percentile die, or a hundred-sided die — is one of several standard polyhedral dice used in gaming. Each type is generally identified by the use of the letter "d" (for "die" or "dice") and the number of sides the die features. Multiple dice are connoted by an additional number prefixing the D. So rolling five hundred-sided dice would be noted by writing 5d100.
Two "types" of d100 are generally used: the most common is to have two d10s. Either they are each of a different colour (with one representing the "tens" digit and the other representing the "ones"), or one of the dice, called a "percentile" die, will be pre-printed with tens (10, 20, 30 instead of 1, 2, 3). A roll of two zeros (0 and 0, or 00 and 0 on percentile) indicates a roll of 100.
The second option is to have an actual d100. Due to the necessity of the size of the die and the small size of the faces, this type of d100 is often nicknamed a golfball, or a zocchihedron (after its creator, wargamer Lou Zocchi). They're often prohibitively expensive and have a habit of continuing to roll until they fall off a table, so golfballs aren't very popular.
Dice Used in Roleplaying Games