For example:

FOR SAVING:

  NSUserDefaults *prefs = [NSUserDefaults standardUserDefaults];

  // saving an NSString
  [prefs setObject:txtUsername.text forKey:@"userName"];
  [prefs setObject:txtPassword.text forKey:@"password"];

  [prefs synchronize];

FOR RETRIEVING

  NSUserDefaults *prefs = [NSUserDefaults standardUserDefaults];

  // getting an NSString
 NSString *savedUsername = [prefs stringForKey:@"userName"];
 NSString *savedPassword = [prefs stringForKey:@"password"];

NSString *appDomain = [[NSBundle mainBundle] bundleIdentifier];
[[NSUserDefaults standardUserDefaults] removePersistentDomainForName:appDomain];