In the earlier versions of Xcode, the Interface Builder generated NIB files (pronounced Nib). These were not individual files, but were actually directory packages. Most developers who worked with these files would probably agree that directory packages are somewhat complicated to work with, especially with regard to version control. With Xcode 3.1, Apple introduced XIB files (oddly enough, also pronounced Nib). XIB files are flat XML files depicting the layout information of the view. Since they are written in XML, they are much easier to tweak (when necessary), easier to track with source control, and they make reviewing changes between versions simpler. Ultimately, the XIB files are compiled into the .nib files prior to deployment, but you won't be able to edit those because they open in Interface Builder as unreadable binary code.

With the introduction of .NET into the mix, Xamarin added DESIGNER files. For lack of a better description, designer...