Everybody knows and understands the quest model. You go to see a guy. The guy asks you to do something. You go and kill the ten rats, or click the object, or fetch the shrubbery, or whatever. You return to the guy. He rewards you and may ask you to do something else. Repeat, with variations, several gazillion times over. Ever since the first RPGs had you encounter the Guy in the Tavern who Needed Something Done, quests and contacts have been a staple of the genre.
But the model is mutating more and more. Warhammer Online's public quests do away with the need for a quest-giving contact altogether. Although you still visit a Rally Master to cash in your Influence, the quest simply applies to an area, rather than being handed out by a contact. Now, Massively has learned that DC Universe Online will offer a great deal of dynamically spawned content centered upon the player, popping up as you move through the city, and though it will still have traditional questing it will generate a great deal of its content spontaneously, through 'encounters' instead of quests.