A user interface file is a type file (a file with a .storyboard or .xib filename extension) that contains the source for the user interface of an app. A storyboard (.storyboard) file contains a set of view controllers and their views, and the relationships between the view controllers. A xib file (.xib) usually contains one view controller or menu bar. The contents of .xib and .storyboard files are stored in XML format. At build time, the .xib and .storyboard files are compiled into binary files called nibs. At runtime, nibs are loaded and the objects they contain are instantiated.