WordPress employs categories and tags as built-in taxonomies for posts and pages. Link categories are a taxonomy for the blogroll or links. So, what do we mean by taxonomy?
The term ‘taxonomy’ makes me think back to high school biology class where we learned about Carolus Linnaeus. He was a botanist from the 18th century who devised a classification system for naming species that is still being used in the 21st century. This Linnaean classification system organizes species of plants and animals into hierarchical levels including Kingdom, Phylum, Class, Order, Family, Genus and Species.
With respect to biology taxonomy is the naming system used to identify millions of different creatures, both living and extinct. All species are classified according to their characteristics into broad groups containing similar organisms. Each broad group, like a kingdom or class, is subdivided into distinct sets, like family or genus. Being a member of a certain group means that you have certain characteristics in common with other members of the same group. Each organism can then be described according to the groups that it belongs to.
So, a taxonomy is a way to describe, identify, name and classify individual species. If we extend this idea to WordPress, a taxonomy becomes a way to organize and classify blog content.
Assigning a post or a page to a category or giving your article a couple of tags classifies and organizes the content. It helps the reader and search engines, too. Same with the link categories, they help us to organize links into meaningful groups.
Categories can have a hierarchy and can be divided into subcategories, but tags cannot. In describing categories we can use the term ‘parent category’ for a larger category that has been divided into smaller groups or ‘children categories’.
Category hierarchy is useful when presenting information about large groups of things. For example, a site about muscle cars can have the broad categories of Chevrolet, Pontiac and Plymouth. Within the Chevy parent category, there might be subcategories of Chevelle, Camaro and Nova. LeMans and GTO could be child categories of the Pontiac parent category, and the Plymouth parent category might list Barracuda and Road Runner as subcategories. You get the idea.
By filing a post in a child category, it is not automatically a member of the parent category as well, so make sure to tick the parent category if that’s what you want.
Taxonomies are searchable so they are a way to help your site visitors find your information. Choose category names wisely at the start. You can still add or subtract categories as your blog develops, but if possible refrain from changing categories, especially if your permalinks, and therefore the URLs of your posts, use categories. It could be a nightmare to update all those links!
Tags are like keywords that describe a post, so they’re not thought out as part of the site structure like categories should be. Tags don’t have hierarchy and are described as having a freeform and one-dimensional nature. Basically, tags are used to further describe or classify a post or page. The same tag can be assigned to any number of posts or pages in any combination of categories.
Using taxonomies is a means for connecting posts together by relationship or similar characteristics. Grouping similar posts into categories and tagging them will help visitors to navigate and use your site.
But WordPress doesn’t stop there, it lets you to create special taxonomies for any of your sites. Custom taxonomies can be defined for custom post types, posts or pages, and have been around since WordPress version 2.3, but they didn’t gain the full capability for hierarchical structure until WP 3.0.
Interested in creating a new, custom taxonomy for your site? Study the codex examples for registering taxonomies.
Categories, tags and custom taxonomies are enlisted as some of the post meta data. Post meta data includes the information about a post that helps to identify it. Examples of post meta data include the date and time it was published, the author’s name, the categories or tags assigned to the post, and the associated RSS feed.