Dungeons & Dragons Online by Eberron Unlimited (abbreviated as "DDO") is a massively multiplayer online role-playing game (MMORPG) developed for the PC by Turbine, Inc. DDO was developed as an online adaptation of Dungeons & Dragons (D&D) based loosely on the D&D 3.5 Rule-set and set in the Eberron campaign setting. DDO was released on Feb 28, 2006 and was published by Atari. DDO is free to download and play, with players having the option to purchase additional content from the in-game DDO store or to subscribe for a monthly fee. The subscribed users gain access to all game content and recieve bonus points to spend in the store. All additional content is optional and the game is able to be played from level 1 to 20 completely free of charge.

DDO is a fantasy role-playing game. Class progression is achieved by completing quests to level up. After creating a character on the character selection screen, players receive quests in-game from non-player characters. After accumulating enough experience points through quests, the character is entitled to gain levels, which grant access to things like new skills, spells, and class abilities.

The controls in DDO are similar to other MMO's in that the character can be moved forward, back, and side to side with the 'w', 's', 'a' and 'd' keys. The mouse is used for zooming in and out, looking around, targeting and attacking.

What sets DDO's combat apart from many games of this genre is the unique "active combat" system. Players click the mouse button to attack or cast spells. Ranged attacks and spells can be dodged or blocked by moving out of the way or using objects as cover. There is no 'auto-attack' as found in many other games.

Character creation in DDO is generally more in-depth than other MMO's, with six races and eleven classes to choose from during character creation, as well as dozens of feats, skills, and spells. As characters advance in level they have the ability to multi-class into 2 additional classes, giving the player potentially hundreds of different class combinations to choose from.

Characters advance by completing quests, which award experience points. After enough experience points are earned, characters can advance in levels. These advancements allow access to new class abilities, spells, feats, skills, and combat abilities.

Quests are completed in instanced dungeons for the most part, and can be set to different degrees of difficulty including solo, normal, hard, elite, and epic. Automatic dungeon scaling will also increase the difficulty of the dungeon's creatures to match the size of your party, so a full party of 5 characters on normal will face the same degree of challenge as a party of 2 characters would.

The learning curve for DDO is not too steep, as your first level will be spent questing in a tutorial town where you learn the basics of controls, spells, combat, and leveling. Though with the amount of feats, skills, and the ability to multi-class it will be a long time before a person really masters all the subtleties of building an effective character.