servers..: I belive the game is designed as a multiplayer game where the person loading/hosting the savegame is the host.

So no matter if you play singleplayer you just play on your own "private server" that nobody can see, and if you choose server so other can see your "singleplayergame" then you become the server automatic.. If you select server you can set a password.
The nice thing is you can use your singleplayer savegame in multiplayer so a friend can jump into your game and help you or you can jump to his.
And its easy to start a server if you want.
If you dont put a password on if you select server then everyone can join.

Renting a server is the same as you hosting a server game your self.
The difference is its just an online server somewhere in the world that "run" the game with its own savegame.