NIB, Interface Builder User Interface File (.nib)
The NIB Developer file format
NIB files are created using the Apple Xcode Interface Builder. This program is used for software development of user interfaces for Mac OS X applications. The Interface Builder is part of Apple Xcode. The NIB file saves all necessary information needed to build a user interface, such as text fields, buttons, a window or view component and other, more interactive components as well.
Technical details of NIB files
Mac OS X applications are able to run without the usage of NIB files. There is, however, an advantage to use them nonetheless. While user interfaces can be developed by programming them, Apple Xcode Interface Builder helps to create user interfaces visually. In newer versions of Interface Builder (version 3 and newer), the NIB format has been replaced by the XIB extension. While NIB files store binary data, XIB files are text-based.