The question of tags vs categories keeps coming every six months or so, so to avoid repeating myself over and over again, here’s an explanation for you all 🙂
For Tags vs Categories, first a brief history …
In the early days of WordPress, tags were not a thing. The idea with categories, was that you would have specific themes, ideas etc. for your site, and you would categorise things based on those. Posts can be a member of multiple categories. Later they add hierarchical categories, so that you could have a parent category of another category.
Then some bloggers started making custom plugins which added another option, “tags”. Initially most people (including the development team of WordPress) thought this was stupid, and the people using these plugins were just idiots who couldn’t understand how the categories system works. They could literally use the categories system for this task, but they insisted on using “tags” despite them being functionally identical to categories expect that they were missing some features. Why use something which is identical to something else, but isn’t as good since it’s missing features? The answer to this perplexed most.
Eventually WordPress and many other platforms caved in and retrospectively added “tags”. The only difference technically speaking, is that they have different names and you can’t have one tag as a parent of another tag.
There is a difference conceptually however. Categories are used for broadly categeorising posts and for when you need a sub-set of a type of posts. Tags are used for arbitrarily labelling posts as a certain type of post. Posts typically have more tags than categories.