Players of D&D invent fictitious characters who embark upon imaginary adventures in which they battle many kinds of fictional monsters, gather treasure, interact with each other, and earn experience points, becoming more and more powerful as the game progresses. D&D departed from traditional wargaming in this regard by assigning each player a specific character to play, as opposed to legions and armies. D&D also developed the concept of a Dungeon Master (DM), the storyteller and referee responsible for creating the fictional setting of the game, moderating the actions of the players' characters, and roleplaying the supporting cast of non-player characters.
A game usually continues over a succession of meetings among the players and the DM, in which case it is called a "campaign". Usually, each player plays one character, although some games allow a player to play more than one character. The players and the DM keep track of their characters' activities, attributes and possessions using paper and pencils, or the electronic equivalent. The published rules of the game explain how to create and equip a character for adventure, how the powers and abilities of the characters work, how interactions and combats work, and describe the many magical spells and items that characters are likely to encounter. However, the rules encourage DMs to modify the rules to suit both their own campaigns and the group they are playing with.
The early success of Dungeons & Dragons quickly led to a proliferation of similar game systems, such as RuneQuest, Tunnels and Trolls, Traveller, and Arduin. Despite this competition, D&D has continued to dominate the roleplaying game industry throughout its existence, enjoying a nearly impenetrable market position. In 1977 the game was split into two slightly different versions: the simpler Dungeons & Dragons and the more complex Advanced Dungeons & Dragons (abbreviated as AD&D or ADnD). In 2000, the simplified version of the game was discontinued and the 3rd Edition of Dungeons & Dragons was released as a major revision of the AD&D game. The current version of the game, released in July 2003, is Dungeons & Dragons v3.5 (also known as the Revised 3rd Edition or D&D3.5).
As of 2006, Dungeons & Dragons remains the best-known and best-selling roleplaying game, with an estimated 20 million players worldwide and over US$1 billion in book and equipment sales (according to a BBC news report). Products branded Dungeons & Dragons made up over fifty percent of the RPG products sold in 2002[citation needed]. Outside of the gaming community, D&D has become a metonym used to refer to roleplaying games in general.