WordPress includes two user-facing taxonomies out of the box: categories and tags. Both are used to group posts. Additional taxonomies can be added by themes and plugins. For example, if you’re running a recipe site, you probably have a custom post type called “Recipe” that contains all of your recipes. In that case, you could add a taxonomy called “Ingredients” to the recipe post type, and make it possible for visitors to sort recipes based on which ingredients they contain.