Dungeons and Dragons
Dungeons and Dragons (D&D or DnD) is a role-playing game which takes place in magical fantasy world of mythical creatures such as elves, dwarves, orcs, and dragons. Players undertake a series of heroic adventures or quests, following a plot controlled by one player called the dungeon master (DM).
Dice of various types are used to determine outcomes. As players' characters accomplish more, they gain experience points (XP) which allow their characters to gain levels, which in turn make them more powerful and have more abilities. The DM may use a commercially-produced adventure, or may create one of his own.
The game allows players to play characters who follow one of nine different alignments; lawful good, neutral good, chaotic good, lawful neutral, true neutral, lawful evil, neutral evil, or chaotic evil. The character's alignment gives a guideline for how he or she would react in some situations. Paladins, for example, are required to be of the lawful good alignment. Most DMs forbid evil characters as they are often disruptive to the game.
The game settings manufactured by Wizards of the Coast (Greyhawk, Faerun, Eberron, etc.) also expressly adopt a polytheistic pantheon of deities and reject real-world religions as a part of the game. That being said, there is nothing to stop a DM from coming up with a monotheistic world in which to run his adventures. In fact, there is an unofficial published setting, Testament, that is about roleplaying in the Biblical Era. Most characters in the game worship fictional deities both as lay persons and as "clerics", a profession than any character may choose to undertake assuming their alignment matches or is close to a match with their chosen god. Clerics of these deities are granted magical powers by their chosen deity by means of which they can defeat their enemies, or heal their party members.
The game has been steadily modified and expanded since the original first edition and is now in edition 3.5.
There is now a Dungeons and Dragons Massively Multiplayer Online Roleplaying Game (MMORPG) known as Dungeons & Dragons Online: Stormreach which takes place in the Eberron campaign setting.
The Magic System
The magic system used in Dungeons and Dragons was adopted very roughly from the writing of fantasy and science fiction author Jack Vance. In the Vancian system, magic-users such as wizards must prepare all of their spells in advance for that day by memorizing them and a spell leaves the wizard's memory upon being cast. To cast the spell again, the magic-user must once again memorize the spell. The Vancian system was chosen for a variety of reasons such as its originality compared to pre-existing magical systems, its avoidance any connection with systems of magic described in any religion or occult theory, and it being the creation one of Gygax's favorite authors.
The Combat System
Players fight by throwing dice having 20, 12, 10, 8, 6, and 4 sides. These dice are properly called polyhedral dice. This is important to distinguish these dice from standard six sided dice. Rolling two 10-sided dice creates a two-digit random number for percentages, which the rules will sometimes refer to as rolling a 100-sided die.
In combat there are three different ways that a character can damage another character: melee, ranged, or magical. Attacking another Player Character (PC) is often an evil act that will get you tossed out that group, but PC-vs-PC combat is sometimes entered by mutual agreement of the players as part of a plot.
Controversy and Criticism
Starting in the late 1970s the game and others like it came under attack mainly from some conservative Christians as promoting occult and criminal activity. Its dangers were summarized in the Dark Dungeons tract by controversial fundamentalist Christian author Jack Chick, who claims that D&D players whose characters reach a high enough level can learn to cast real magic spells.
Player characters are sometimes rewarded with treasure and occult powers for summoning and worshiping demons and devils (but this depends upon the dungeon master and alignment of the character - in other words it never has to occur).
Another criticism is that illustrations in the rule books for Dungeons and Dragons often contain images of immodestly dressed women, and, in fact, in the first edition Monster Manual a number of pencil drawings were topless (such as harpy and succubus). When the drawings switched to color many years ago, topless pictures were removed.
Tracy Hickman, one of the main authors of the Dungeons and Dragons DragonLance book series, and a Christian with conservative politics and theology, has written a number of articles defending and discussing D&D from a Christian perspective.  Others within the Dungeons and Dragons community responded by writing other defenses from rationalist perspectives or other perspectives or by writing parodies such as "Chess: The Subtle Sin: Should Christians play chess?". In response to the perceived Christian persecution of the Dungeons and Dragons, darker themed, deliberately counter-cultural games appeared in reaction such as Call of Cthluhu which is based on the horror writing of H.P. Lovecraft and set in the Cthulhu Mythos, or Vampire: the Masquerade, where players acted the roles of undead blood-suckers.