To save data even after the app is killed/terminated, you have to use persistent storage. There are a number of different ways to do this. A few of these ways are using NSUserDefaults, plists, or CoreData.
See these links, and try to find out what's best for what you're trying to do.
iOS persistent storage strategy
Use SQLite, plist, or something else?
You should also check out the iOS development lectures, there's two parts and the first part covers persistent storage in great detail. It's called Effective iPhone App Development. I recommend watching both, but the first part of the first one would answer your questions.