DDO Dungeons and Dragons Online Quests - Dungeons
There will be multiple quests associated with each dungeon, and the dungeons will undergo changes depending on the quest you're doing. For example, the first time you raid the kobold fortress, you might catch the kobolds by surprise. The next time you go there (say, to capture their leader), they might very well be ready for you, and you'll have to overcome numerous traps and well-organized defenders in order to accomplish your goal.
When you go into a dungeon, you will always be on some kind of quest, so there will be some kind of plot-driven context for your dungeon crawls.
Dungeon quests generally have one or more primary objectives, and can have a number of secondary objectives as well. Only the primary objective(s) needs to be completed in order to successfully finish the quest. Completing secondary objectives will increase the experience you receive, and possibly reward you in other ways as well. Experience will scale based on your level -- if a dungeon is considered easy for your party's average level, you'll get less experience for completing it.
Some quests will be repeatable, and some won't. Similarly, some quests will have random elements (monsters, traps, placement of quest objectives, etc.), and some will be mostly fixed.