Categories give you twelve big rooms. Tags are how you find the corner you actually wanted: a genre, a mood, an engine, a tool. Makers apply them to their own games, so they describe what the game is rather than what it was marketed as.
What the game is. These overlap with categories on purpose. A category is the one shelf a game sits on, a genre tag is every shelf it could have gone on.
How the game feels. The most useful tags on the site and the hardest to fake, because a maker who tags their game cozy and ships something brutal hears about it in the comments.
What it was built on. Engines, renderers and browser APIs, declared by the maker at submission.
What helped build it. Mostly AI assistants, treated here as a plain fact about the process. There is a fuller breakdown on the tools page.
Use a category when you know the shape of what you want: an hour of strategy, twenty minutes of horror. There are twelve of them and every game is in exactly one, so a category page is always a full page.
Use a tag when you know the flavour. Tags cross categories freely, which means a tag page can hand you a roguelike from Action, one from RPG and one from Experimental in the same grid. That is the whole point of them.