From the introduction:
Welcome to Dungeons and Dragons Online or, as it is often called, DDO. We hope you will enjoy your stay here. There are a lot of choices to be made before you start and many more during your time here. Choose wisely for they will impact your journey and what you take away.
There are a large number of class and race combinations for you to pick from. Quests and dungeons are plentiful here and not all of them are your typical hack and slash adventures. The possible levels attainable are from 1 to 20 in various classes and up to 10 epic levels.
Only three classes are able to be used simultaneously. Reincarnation is also possible to gain more power by offering unique bonuses from Heroic, Racial, Epic, and Iconic Reincarnation.