The auction house is a system that lets you buy and sell in-game items to and from other players. In order to access the auction house, speak with any of the Auctioneer NPCs in any of the major towns.
This window will allow you to find auctions for a specific item. All you will need to do is to figure out the category and subcategory that it is listed under, and then click the Use Filter button.
This tab will allow you to monitor the status of any auctions that you have placed bids on. Listings shown in gray indicate that you have been outbid for this item. Listings shown in white indicate that you are currently the highest bidder.