[iOS] viewDidLoad viewDidAppear viewWillAppear

viewWillLoad/viewDidLoad - called when the view is constructed (via the first call to retrieve the view controller's UIView via it's view property - aka lazy loading)
viewWillAppear: - when the view is being prepared to appear either immediately (animated == NO) or view a transition (animated == YES)
viewDidAppear: - if the view appearance wasn't cancelled and the view controller's view fully appears
viewWillDisappear: - complements viewWillAppear:
viewDidDisappear: - complements viewDidAppear:

viewWillUnload/viewDidUnload - deprecated APIs when the view is unload due to memory constraints (don't worry about these anymore)

Source : http://stackoverflow.com/questions/22214843/ios-7-difference-between-viewdidload-and-viewdidappear