Obviously there is openings.

There is checkmating patterns and other tactical patterns as well.

I've looked at the pawn structure videos on here, but honestly I haven't been able to gain much from watching them yet.

What other topics are there?

Topics to study  are conveniently located in your played games, specifically your losses.

Show a reasonably strongER player your losses and he/she will be able to make a few excellent suggestions of things you need to work on.

 Hint: Openings are NOT going to be one of them as they offer the least bang for your buck if your rating suggests your current true playing strength.

I'm not talking about which things people feel you should focus on at which times.

I'm more asking how many sub topics that are studyable as themselves can chess be broken down into.

If you still want an insightful answer please let me know and I will post what I hope you will find very informative.

chess sponge, the answer to your question as I understand your question is nearly infinite.

tactics: forks, skewers, pins, double attacks, clearance sacrifices, removing the defender etc.

Strategy: Openings, Knight vs Bishop, Bishops of opposite colors, Good and Bad Bishops, Pawn structure, isolated pawn, pawn islands, backward pawn, passed pawns etc.

Every time I think I have figured something out I am learning that I have a lot more to learn about this game.

It's nearly infinite as you could decide to focus in further and further and turn everything into a subject. You could study, for example, games where GMs put knights on the rim. You could study the relationship between position and tactics for the queen's knight. Basically any question you want answered is a specific topic to study.

The big ones are tactics, middlegame strategy, endgames, and openings.

Maybe I'm just not being clear.

forks, skewers, pins etc. Those are tactical patterns. Tactical patterns would be a topic of study which would include all of those things.

isolated pawn, pawn islands, backward pawns etc. All of those would be part of pawn structure.

Sicillian, English, Caro Kann, etc. are all openings. Openings would be the topic of study.

So I've listed:

 Tactical Patterns (with checkmating patterns being an important enough sub category to get its own listing)

Pawn Structures

Openings

What other categories are there, not individual very specific examples?

Typically you see books written as Opening, Middle, and Endgame. I feel that Middle game is probably too broad of a topic and should be broken down more.

you can study endgames which is different from endgames puzzles, Another topic would be mating pattern.

I would say endgames, how to attack and how to defend. Those three would fit in with the other broad categories you listed. 

Well the responses in this thread have concluded for me that there are only 4 general areas to chess, possibly 5 depending on how you want to classify things.

Openings

Tactical Patterns

Pawn Structures

Positional Patterns

and possibly Endgames in its own category even though that would be a combination of tactical and positional patterns.

Not really what I was hoping for, but I guess it is what it is.

thinking process

time management