What’s the difference between Camera Roll and Photo Library?

From When should I use UIImagePickerControllerSourceTypePhotoLibrary instead of UIImagePickerControllerSourceTypeSavedPhotosAlbum?:

UIImagePickerControllerSourceTypePhotoLibrary references the entire photo library, letting the user choose which album. UIImagePickerControllerSourceTypeSavedPhotosAlbum goes straight to the camera roll album without giving the user a choice as to which album to choose from. They’re similar, but different. You can get to the camera roll from PhotoLibrary, but you can only access the camera roll from UIImagePickerControllerSourceTypeSavedPhotosAlbum.

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