Creating custom taxonomies in WordPress revolves around a WordPress function called register_taxonomy(). This function is passed an array of parameters that determine properties of the taxonomy, and then hooked to WordPress's init event.
In some situations, you may want to programmatically register taxonomy terms. This is done with a function named wp_insert_term(), which also hooks to init.
For a given taxonomy and a given post, you can print out the taxonomy terms that apply to that post using a function called the_terms(), and you can fetch (but not print) those terms using a function called get_the_terms().
This chapter will explain how to create custom taxonomies, and present an example. To understand why and when to create a new custom taxonomy, check out the previous chapter, Organizing Your WordPress Data: Understanding Custom Taxonomies and Custom Fields.
How to Register a New Custom Taxonomy
The key function for registering a taxonomy ...