The wardrobe is where all of a character's outfits, hair ornaments, and other clothing pieces are stored for your character (and if you are a summoner, your familiar as well). It can be accessed by pressing F3.
The wardrobe also allows users to preview all clothing for any race and gender. It also includes details on how and where to obtain the costumes. However, the wardrobe storage feature is only available for Premium Member, which means non-premium player cannot store their costumes in it. They can still use the preview.