A somewhat
strange die, the d10 is an
polyhedron whose sides are composed by
identical but
irregular quadralaterals; two sides of each
face are short and connected by an
obtuse angle while the other two sides are long and connected by an
acute angle. Five of these
acute angles converge at each
point of the die, while the
obtuse angles
interlock in the
middle.
In any event, a d10 is generally numbered from 0 to 9, making it the only kind of die that usually has the number 0 on it. When instructed to roll a d10, though, the 0 is treated as if it were a 10. This seems strange until one learns about rolling d100; in this case, one die is the tens place and the other the ones place, and rolling two zeros is considered to be the same as rolling 100.
While this die is used in the various forms of D&D and AD&D, among other RPGs, its real claim to fame is its exclusive use in White Wolf's World of Darkness games. If that setting appeals to you, invest in a bunch of these.