I want anyone who plays my game, to save everything they've done in the mmorpg when they log out or are kicked off. I would expect a hard save system. So i probably would have to code as if the cant change anything they do, right? Can you tell me how to do this in a mmorpg on unity? I may need everything you know about it.
Well a traditional MMORPG implementation requires a separate server which the user interacts with to get information about the locations of other players and NPCs.
This is used to avoid cheating which would be inevitable if you didn't have an authoritative server. This means that the server, not the client holds all the information about your massive world, including all character positions and stats and all monsters and special items.
By keeping all that information on the server, your job of saving is much easier as you will do it continuously. If player A kills a monster and acquires a new sword, the server must know this and report it to Player B so they can see the monster death animation and also see Player A's character holding the cool new sword.
That information would then be saved in a database as it happens so that if a player quits, their state will be persisted.
The site How stuff works wrote this subject up and provided a nice visual walkthrough of the logic.
DevMaster.Net also has a nice article which focuses on the big picture decisions for a small team developing a MMORPG
FYI - If you're early on in the process, it is probably a good idea to host the server, yourself for cost reasons. Nevertheless, if you are planning on a large user base and you don't have a lot of cash, you may consider looking at Amazon Web Services which provides server and database hosting that can grow automatically as your usage grows.