QuestString

Quest string and QuestLinks were added in patch 2.4.

Quest String Formatting
Quest strings consist of the quest id and the quest level.

quest:questId:questLevel

Note that the QuestID is actually used by wowhead and probably other sites - you can actually go to a quest page on wowhead, and change the end of the url to the in-game id, and you should get the page for that quest.