How can you store data between application launches?
There are plenty of ways, but here I will show you the simplest, NSUserDefaults.
NSUserDefaults is a simple property list (aka plist) where an app can store simple data. While there is no limit to its size (besides the device’s own limits), you should not store a large amount of data here. The file is written and read atomically (as a whole), so the more data that is in there, the longer that will take. Nonetheless, it is a great place to store settings, high scores, and the like.