There are two main taxonomies used in blogging: categories and tags. Categories are what they sound like: top-level, broad categorisations usually referring to the type of content. Categories are arbitrarily named and differ from blog to blog. For example, a political blog might have categories like “News”, “Opinion”, “Investigative Reporting”, etc., while a sports blog could have categories like “Football”, “Basketball”, “Hockey”, etc.
At a more granular level are tags. Tags are arbitrarily named keywords that are generally used to indicate that a certain topic, person, company or thing has been mentioned or referenced in a post. This aids readers and researchers looking for information on a particular topic find posts that match. The sports blog, for example, might tag a particular player or coach in every post mentioning their name, and then readers could find all posts relating to that player or coach in the one place.
While typically a post will belong to one or very few categories (because content generally only has one type), conventionally tags are much more liberally used and a single post could potentially have dozens of tags depending on how many things are referred to in the post.
Categories and tags became extensively used on the internet with the rise of blogs (and in particular the Web 2.0 era of the early 2000s), because they provide a handy way to categorise and search large volumes of information without having to perform a resource intensive full-text search. The general idea of categorising and tagging has existed in bibliographies for centuries, but its particular digital form has only come into widespread use in the last decade or two.
The WordPress knowledge base has a more detailed explanation of the difference between tagging and categorising.
Tagging isn’t necessarily limited to blogs. On the film diary service Letterboxd, where I often post short reviews of films I watch, tags are used extensively by users of the site in a number of ways. Tags like watched-at-the-cinema or watched-on-netflix indicate the circumstances in which a film was watched, and tags like 52-films-by-women are used by people participating in group-watching marathons and challenges. This particular tag helps bring together all of the people participating in a challenge to watch one film directed by a woman each week of the year, enabling them to meet and engage with each other through Letterboxd.
I use categories and tags extensively on this blog. I use categories to split up my posts according to the subject for which they’re created, such as the Networked Media category. I also have tags like blog-checklist, which collects all of the posts that constitute the first Networked Media assessment task.