The quests in DDO are organized by character level, difficulty and length. Each quest has a base level, and the XP reward increases if all of the party members are below that level, and decreases if any of the party members are above that level. Characters more than three levels below the "base level" are not allowed to initiate a quest, however they are able to join a quest started by another party member.
Note that most quests have an item or monetary reward when finished, but the character must have talked to the quest giver to receive the reward. Some dungeons require several visits to the quest giver to complete the entire quest chain. Speaking to the quest giver also generally allows you to repeat the quest.
The quests are narrated by a "Dungeon Master" who gives additional details and information to the players about what they see and hear on their adventure. For the "Delera's Tomb" quest chain, the dungeon master duties are performed by (now deceased) D&D co-creator Gary Gygax.