I'm somewhat confused as to how Go strategy works. Is it mostly protecting your own groups while restricting the development of your opponents?

I'm assuming that there's still opening, middle/endgame. What are the general strategies of each stage?

Go is a game of profit. You need to find ways to make points while your opponent isn't.

There's an old joke -- Two campers are out in the woods, they woken up in the middle of the night by a rustling sound outside of their tent. The camper #1 says "Shh...I think it's a bear!" as he slowly starts to reach for his athletic shoes. Camper #2 asks "Do you really think you can outrun a hungry bear?". Camper #1 replies "No, not at all....but I'm fairly certain I can outrun you".

Go is very much the same way...some people get caught up thinking they have to try and outrun the bear. They focus on playing what they think looks like the absolute biggest move all the time...but the problem with playing the absolute biggest move is that there's always an absolute second biggest move...and it's usually about the same size. If you play a move that claims 50 points, and your opponent follows up with a move that claims 49 points....the end result is that you gain 1 point.

If you play a move worth 10 points, but you can do it in such a way that your opponent must respond defensively (not really earning any points) then you have only made 10 points...but you've actually made a large gain because of the differential (10 pts vs. 0 pts).

With that in mind...I like to think of Go in terms of weak groups (essentially things that potentially might die). Weak groups are how you profit, because you can try to make point while attacking them, and hopefully your opponent will be making non-point making moves (e.g. moves that are used to run away or moves that are used to make eyes).

This relates to the phases of the game :

-The Opening: Groups of stones aren't really big enough or well defined enough to get classified as "weak". If a single stone gets cut off and killed, you will likely find a way to be compensated for it. You are loosely outlining frameworks, and making groups that are prepared for middle game.

-The Middle Game: All about attacking and defending weak groups. Groups exist that have enough structure to be important and require saving, but not so much structure that life and death is completely sorted out.

-The Endgame: Groups are mature enough that all major matters of life and death have been sorted out. The boundaries are more or less defined, but you are now fighting over their exact placement. Usually at this stage of the game it is possible to count out exactly how many points a move will gain or take away, as opposed to trying to make a vague assessment ("This makes me more stable").