Items come in three forms:
Equipable Items[]
Equipable items provide either stats or aesthetic improvements. Some can also be used to gain access to secret locations. They also have a few categories.
Consumable Items[]
Consumable Items have quite a few uses also. From Healing the player to providing Experience and even Changing the Player's name. You can find all their uses on their individual pages.
Static Items[]
Static Items are items that don't fit in any other category and are generally mundane or only used in crafting.