A quest chain or "chain quest" is simply a series of quests, which must be completed in a particular order.
The name of the quest will sometimes be repeated, creating ambiguity. To resolve this ambiguity, WoWWiki adds parenthetical numbering to quest names. (i.e. Battle of Hillsbrad (2), Battle of Hillsbrad (3), etc.)
Quest chains sometimes converge or diverge because of faction differences. For example, the Dungeon Set 2 chain diverges into Anthion's Parting Words (Alliance) and Anthion's Parting Words (Horde).
Quest chains will normally give better rewards than normal quests.
See also: Category:Quest chains.