You mean all those funny looking dice? What about when people say "d8" or "2d4 +2"?
Alot of dice are used in Dungeons and Dragons used by both players and Dungeon Masters. This is to provide unpredictabilty and randomness to the game. When some asks you to roll a "2d4 and add your damage" don't panic. It just basically means to roll the 4 sided die twice, add the totals and then add your damage your character. Below are some explainations of uses of the dice and a video to help.
For example, if you had to search for a loose brick in a dungeon wall you would have to perform a search check. The characters have to get a total of 5 or more to succeed (Search DC = 5).Two characters could roll at the same time...but Ralik the Rouge has 4 points in his search skill whilst Warick the Wizard has 0 points in his search skill. The formula would proceed like this: d20 + (skill or attack bonus) = Total. In this scenario both characters roll a 2 and they add their search skill to the roll:
Ralik the Rouge = 2 + 4 = 6 *SUCCEED*
Warick the Wizard = 2 + 0 = 2 *FAIL*
Therefore, Ralik finds the loose brick whilst Warick fails miserably.
12 sided die or "d12"
10 sided or "d10"
Used to find out the chance of something happening with regards to a percentage. This should not be confused with a skill roll. An example of this use would be to suggest a "25% chance to encounter wolves when travelling in the wilderness". You would roll 2 lots of d10 (2d10) to find a percentage.
8 sided dice or "d8"
6 sided dice or "d6"
4 sided dice or "d4"
A pyramid looking dice that is mainly used in some wizard spells such as magic missle. Used for some weapons such as a dagger.