DDO is set on the fictional continent of Xen'drik, in the world of Eberron. Eberron is a campaign world, developed by Keith Baker for Wizards of the Coast. Players can create their characters following the revised edition of D&D 3.5 rule-set fashion, and play them in both indoor and outdoor environments, including a large variety of dungeons.
Although the game is based in large part on the D&D 3.5 rule-set, it contains numerous changes from the tabletop game, most of which were introduced due to differences in the dynamics between the media of computer game combat and tabletop gaming. For example, Turbine wanted DDO to use a real time combat engine, where tabletop D&D uses a turn-based system. This brought considerable amount of changes in combat, character skills and feats, situations where Turbine felt the turn-based combat system and real-time combat did not mesh well. Most noticeable differences are greatly increased amount of hit possibilities in round/same amount of time (as much as twelve times more), greatly increased spellcasting rescources over rest periods, spellpoint system instead of spellslots, magic items are greatly underpriced (average of 1/8th the D&D prices), magic weapons and armours have only two abilities, characters have considerably higher stats (as much as +9) and last but not least, offensive effects created by characters does not harm their allies. Near complete list of other differences between DDO and D&D have been listed on the DDO Wiki.
The core of the game is set in the city of Stormreach, a giant-scaled city built eons ago, and recently settled by humans. There are a number of areas within Stormreach - the Docks, the Harbor, the Marketplace and the four Dragonmarked Houses - House Deneith, House Jorasco, House Kundarak and House Phiarlan. In addition, there are a number of adventures that are taken beyond the city walls, into distant lands such as Tangleroot Gorge, Threnal, Three Barrel Cove and Sorrowdusk Isle, each home to one or more adventures of their own.
Generally, the plot of the game revolves around the following dark plots:
  Giants attempting to regain mastery over the city of Stormreach and the continent of Xen'drik
  Cultists attempting to reopen the gate to Xoriat
  Minions of Khyber (Eberron), attempting to gain power over Xen'drik
The game, like many mmorpgs, requires a monthly fee to play ($14.99 USD). There is a ten day free trial available on the game's website.