I had this exact conversation recently. The category / tag / topic can be a little arbitrary, depending on what platform you're using. Wordpress has a hierarchy of category > tag: https://en.support.wordpress.com/posts/categories-vs-tags/
@Phil_Vallender is correct, HubSpot has the topic feature, which is a close analog to a tag. I like to limit how many topics I include on a blog because it keeps you on point, content-wise. Limited tags create a standing reminder to consider the post's target persona. This keeps you from over-complicating your writing.
Last thought: if your blog's layout includes a module with your most popular topics (it should be included in your default template), then those topics, which are likely a keyword, appears on every page using that template, including blog posts. Which means if you have 500 blog posts, and 500 indexed pages, your top 5 blog topics appear on all those 500 pages, strengthening your domain's SEO for those words.
Something to chew on!
-- Alex Crumb | HubSpot Academy
I have a suggestion for a change to blog topic tags. It would be very helpful if there was a way for a specific tag to show up first in the list or sequence of tags. As it currently is, when you can select several tags it randomly pulls one of them to appear in our website's blog template as the primary tag that appears (in a "bubble" area below our headlines). We'd like to have the ability to list several tags, but force one of them (i.e. the first one selected) to be the one that appears in the featured/bubble area.
Thanks a lot, SecureWorldExpo.com