The Auction House (usually abbreviated the AH by players) is a the place to go to find or sell weapons, armour, trade goods, recipes and reagents you need or by allowing you to trade items for gold with other players, the Auction House helps you raise money for that elusive Mount or the every increasing expensive . You can post your items for sale and others will be able to search them up and purchase them from you. Once you've posted your items they will remain on the AH until the auction duration is over, or someone has bought the item for the buyout price. If they don't sell, the items are returned to players by mail.
By enabling you to search for rarer items that are harder to farm, as well as items that you may not be able to acquire yourself such as materials from gathering professions you do not have, the AH plays a vital role, particularly concerning professions.
However, for every positive there has to be a negative and the negative side of the AH is that the Auction House fees and deposit costs are one of the largest gold sinks in the game.
Most cities and some towns have their own Auction Houses. All Auction Houses in a realm are connected, and offer the same items for sale, regardless of location or faction. (In earlier versions of the game there were separate Alliance, Horde and Neutral auction houses). Players with trial accounts can see the items listed for an auction but cannot buy or sell through the Auction Houses.