The typical 12-sided dice (which geometrically is a regular dodecahedron) is used to generate random numbers between... surprise!... 1 and 12. The numbers are typically placed on each die so that the sum of the numbers on opposite faces is 13. There are also specialized d12s that show different symbols, words, or numbers on each face.
In roleplaying games and board games, the d12 isn't as commonly used as some other polyhedral dice, such as 10-sided dice and 20-sided dice, but it keeps a place among the standard 7-piece dice sets that are used for D&D and other roleplaying games. The 12-sided dice is particularly useful in math applications and is a good addition to any arsenal of educational materials.