You would have to ether have single players commands and use /world exit. Or go to the windows task mangier and end the process javaw.exe.
If the game pauses which it does automatically when it loses focus it saves. To be honest, I personally use a Subversion to control changes made to my world. Hell even if you just do a straight backup, these days you can have a large map and it is still be like less than 1mb unlike the old days the same size maps could be a dozen mb big. Just imagine how big my backup folder with a 2 or 3 dozen of 15mb backup files.
It's not possible to go play a previous saved version of your world, and not possible to exit your Minecraft world without it saving. Whenever you press escape it saves automatically.
However, if saved your world, copied the world, and then saved the world again, you could always delete your current world and replace it with the older one by going to .minecraft -> saves and pasting your world in a folder in the saves folder.