NSTimeZone: what is the difference between localTimeZone and systemTimeZone?

Under NSTimeZone class, there is both +localTimeZone and +systemTimeZone. I did a test on iphone simulator, both return NSTimeZone object indicating the same timezone. What is the difference? Which one I should use to find out the timezone setting of the iPhone? Thanks

My test:

NSLog(@"Local Time Zone %@",[[NSTimeZone localTimeZone] name]);
NSLog(@"System Time Zone %@",[[NSTimeZone systemTimeZone] name]);