Persistence: Saving data to a place where it can be re-accessed and retrieved upon restart of the device or app.
Necessary for any app that wants to store data log term and keep it available to the user.
NSUser Defaults
-essentially, a persistent dictionary
-user preferences
-saves data in plist file
-data, string, number, data, array, dictionary
-keep NSUserDefaults < 1MB
1.Store thousands of notes, images, and geolocations
2. Store m4v files
3. Store TableViewCollectionView user preference
