Quest Chains are quests of a series, where completing the first gives access to the next, regardless of reputation, level, or other factors.
Many sites limit quest chains to those cases where an NPC immediately offers a new quest upon completion of a previous one. This strict interpretation, though, misses those quests where the NPC performs some cut scene action or dialog. It also misses links where completing a quest for one NPC immediately enables a followup quest from another NPC.
This page collects all sorts of quest chains. Two linked quests or twenty, as long as it is of interest to someone.