saving?

I believe so. It saved whatever changes that you've made to the existing saved files. I normally just click save every time unless I want to create a different saved file, only then I click save as.

ETA: I just realised that this is TS3. I was talking about the TS4. My bad.

If you have just saved the game and then wish to quit, there is no need to save again in such rapid succession -- unless you want to bump your current one automatically made backup save and have two almost identical ones. "Saving the world" and "saving the game" really mean the same thing, but the former does sound a bit more heroic I guess.

It's a much better practice, before thinking about quitting I mean, to use Save As and change the name of the save file a little bit like by appending a number or the date on the end so that at any given point you have several game saves from various points in time to revert to should something go wrong. Relying on the one backup the game will make if you overwrite every save isn't enough of a safety net for most of us. After a while you will accumulate more saves than you would ever need and can of course archive or delete the oldest ones.

I must have over 150 saves of my one long-running 8 year old game by now (yes, I am that insane). Not all in the user game folder, that would be overload. But they are stored in so many different places and not all on one computer that it would be almost impossible for me to lose the entire game and all of my sims should some bunch of unexpected things happen. Plus sometimes it's fun to load up a very old save and look back on how things were going so many years ago. :)