I am not able to get NSUserDefaults concepts properly. Is that a database of plist file ?
It's a key-value store that is persisted between restarts of your application. How it's implemented has little bearing on how you use it.
I don't know if it's different on the iPhone OS (which i don't believe) but in Mac OS X is it a plist file in your Library/Preferences folder. Pretty sure it's the same on iPhone OS/iOS
来源：https://stackoverflow.com/questions/3178115/iphones-nsuserdefaults-is-a-database-or-plist