A d6, or six-sided die, is a die that has an equal chance of rolling the integers from 1 to 6. Almost all d6s are cubes. The d6 is by far the single most common type of dice in the world, both in role-playing games and in other fields.
One d6 is included in the standard 7-dice set used in games such as Dungeons & Dragons. However, players are generally expected to own more d6s than any other sort of die (they can be bought separately), and many games rely on this and require large numbers to be rolled at once in dice pools.
A d6 is generally a cube (or hexahedron, one of the Platonic solids), with the numbers on its sides represented by either pips (the number shown equal to the number of pips or dots) or by a numeral. (Dice other than d6 tend to use numerals.) The numbers on opposite sides of the cube usually sum to 7.
Cube d6s are easily stacked and packed, and widely available, making them some of the most convenient dice to use for gaming. They are even available in different sizes, which some games make use of (such as Mythender's large Mythic Die).
Some game systems use only d6s for all their resolution. This includes the eponymous D6 System and most games that are Powered by the Apocalypse, which follow Apocalypse World's method of having all rolls be 2d6 + a stat modifier.
D6s are also commonly used in games with alternative means of random resolution. For example, Dungeons & Dragons generally uses d20s for resolution, but d6s are still used widely, including in character creation (to randomly determine a starting ability score, with a value from 3 to 18) and in damage rolls (notably in the fireball magic spell).
- d6 at White Wolf Wiki