I just signed up for Playstation Plus, mostly so that my games saves are safe. That's a lot of time and energy that I don't want lost! I assumed that it backs-up all my game saves automatically, however, after I signed up, I started playing a game, I was prompted to enable backing up the game's save file. Are my games saves only backed up to PS+ for games I'm actively playing? Are my old games saves available?
For PS3, short answer is no. It will not back up every save you have on your system. When you play a game, you'll get a pop up asking you if you wish to back up the game's save to online storage.
Also, it doesn't work for PS1, PS2, PSP, and MINI saves and you have only 1024 MB of space for saves, so depending on how large these saves are, you might not get them all. If your Playstation+ subscription lapses, they'll also only keep them for 6 months.
You can manually select saves you wish to backup, though. On the XMB, navigate to GAME and then SAVED DATA UTILITY (PS3). At the top will be ONLINE STORAGE where you can browse the saves that are currently stored online and delete/restore them. Underneath are all your saves. If you press triangle on any of these, it will bring up a side menu. The easiest way to select games you want to backup saves for, is going up to COPY MULTIPLE, then you check the games you want backed up.