Hats belong to the so-called "Collectibles". These are items that have no physical in-game object, but can be activated either from the Collectible menu, or the Quickslot menu. They can be unlocked by either getting in-game achievements, or through purchasing them via Crowns, Crown Gems or Seals of Endeavors.

Full list of all "Hats" that are available in ESO (Elder Scrolls Online). "Hats" are added to your collection and are unlocked for all characters on your account. Hats will override all your character's attire visuals and physical features.