What does a taxonomy comprise and how does it work? Professional taxonomists may speak of “terms,” “nodes,” or “labels,” whereas most other people with a basic understanding of taxonomy might refer to “tags” or “categories.” A category is a well understood concept, and social media sites have made the notion of “tag” well known.
In addition to the different professional level of such jargon, there is also a distinction in meaning. Ironically, it’s the professional terminology that is vague and the layman terminology that is more specific. Taxonomy “terms,” “nodes,” or “labels,” are all pretty generic and can all have various applications for different kinds of taxonomies, both for broad categorization and for specific indexing. “Tags” and “categories,” on the other hand, each tend to have distinct meanings. It’s not so much what they are, or even how they are organized, but rather how they are used.
Tags are for tagging.
That seems obvious. As for what is meant by “tagging,” that implies you put a tag on something. In fact, you can put more than one tag on something, and that’s typically encouraged in tagging. “Something” is typically an electronic file of some form of content, a document, image, video, database record, blog post, etc. Tags tend to be a brief label indicating what something is about. Tags can be very specific or relatively broad. Information professionals might prefer to call them “index terms.” An organized, alphabetized list of tags could serve as an index.
Categories are for categorizing.
This can also be called grouping or classifying. It implies putting something into a category, often represented as a file folder, whether an actual electronic folder path, or just a depiction of a folder icon. While categories have different levels of specificity, the name category implies a collection of things, so there is an implicit understanding that categories don’t get too specific. An organized structure of categories typically constitutes a hierarchical taxonomy.
Can something go into more than one category? In physical folders no (unless you make photocopy of the document for each folder), but in the digital world, often the answer is yes, but not always (again requiring the copying of files). It depends on the system, and it may involve some workaround. Even when it is possible to put a content item into more than one category, unlike tags, it is still preferable to have most content items assigned to only one category and a smaller number of them that may belong in two categories. For example, there may be a breadcrumb trail for the hierarchy of categories, and the breadcrumb trail may only take a single path. The idea is that the categories retain distinct meaning and usage through mostly distinct content.
Tags and categories together
Because tags and categories are different, it is possible to have both at the same time, especially if the categories are deliberately kept broad and the tags are relatively specific. Content management systems and digital asset management systems increasingly offer features of both categories and tags for managing content. In these cases, the challenge is to decide to what degree of classification to use the categories and to what degree to use the tags. That's exactly what I have done as a taxonomist on two recent consulting projects.
For the amateur taxonomist and indexer, one of the most common exposures to tags and categories is through blogs. Blogging software may permit the blog author to assign a tag or category to a blog post. Whether the tags and categories are appropriately named and used is another issue, though. Blogger.com provides only one option, which it calls "Labels" and utilizes an icon for a tag in the blogging interface, but then displays them when published in the right margin under a heading called "Categories." No wonder my "categories" don't look good; I had created them as if they were tags. Furthermore, the very specific subject matter of "The Accidental Taxonomist" blog makes its posts more suited for tagging than for categorizing. WordPress, on the other hand, gives the blogger both tools: tags and categories. If “The Accidental Taxonomist” blog eventually moves, you’ll know why.