What are Storyboard Files?
Storyboards are essentially a next step in XIB file's evolution. Where XIB files usually contain a single view, storyboards contain more views (sometimes even all the views for your app), along with information on how the user will navigate between these views, in form of "segues".
The .storyboard files you are working with when designing UI are, similar to XIB files, simple XML files that the Xcode designer presents to you graphically. When your app gets compiled, storyboards get broken down into individual .NIB files, just like XIB files do.
So whether you use XIB files or Storyboards, at runtime your application always contains .NIB files.