WoWHead (http://www.wowhead.com/) used to be the primary online resource for players of World of Warcraft, and it still is if you continue to play the most up-to-date retail version of the game. It has become much less useful however for players who continue to play the older version of WoW, such as Wrath of the Lich King.
Some comments on old quests are still useful however, up to a point.
There is an add-on available called LightHeaded, that makes all the quest comments from the 2009 WoWHead website available in-game. This is an tremendously helpful add-on if you intend to complete the Loremaster achievements in the game.
A good alternative to WoWHead for anyone still playing Wrath of the Lich King is http://wotlk.openwow.com/.