There’s a lot of confusion around them, as they’re both terms that you assign to posts so that your readers can find them. Here’s a simple explanation of the difference.
- Categories define a taxonomy to your site, meaning they’re hierarchical, (like folders within folders).
- You should only use one category per post.
- Categories can be nested, so you might categorize something as Linux, which lives under Operating Systems.
- You should have as few categories as possible for your site overall, as they’re high-level organizations of your content. You don’t really want to have a category unless you have more than 20 or so posts under it, as a minimum (for a large site).
- Tags define a folksonomy for your site, meaning they’re organized in a flat structure.
- Tags are meant to supplement Categories by adding more detail. So if you have a post categorized as Linux, you may add the Kernel, Iptables, and Logging tags.
- You are likely to have far more Tags than Categories, because tags are more detailed and specific to the content.
- You shouldn’t use more than 2-3 tags per post (and only one category)
To get the maximum benefit of this organizational structure, you want to assign one category and two to three tags for each post.
For many sites this will require a thorough revisiting of existing categories, existing tags, and their assignments to your posts. Putting this work in, however, is likely to make it easier for both visitors and search engines to find your content.
Hope this helps.