Recipes are items in Arkandian Crusade which teach you the recipe for crafting an item. They are used automatically when bought or looted. The recipe learned is recorded in your recipe book, which can be used to automatically fill your crafting matrix with the required components to craft an item.
Having a recipe is not necessary to craft an item; it is entirely possible to stumble upon a correct recipe through pattern recognition or just plain guessing. In this case, the recipe is automatically added to your recipe book.