Nibs are files (or bundled documents) created by Interface Builder. "Nib" stands for "NeXTStep Interface Builder" and they are usually stored inside the application's bundle, such as in the location AppName.app/Contents/Resources/ or AppName.app/Contents/Resources/xx.lproj/. Nibs contain parameters for creating windows, dialogs, menus, and controls. Nibs largely replace the older Resources format.
Basic Nib support is possible in Carbon programs using the routines from IBCarbonRuntime.h, which is in the HIToolbox framework (covered when the umbrella header of <Carbon/Carbon.h> is used).
Interface Builder provides checks for compatibility for nib files among different versions of Mac OS X.
Some interfaces can be implemented entirely by a Nib, with all code written by Interface Builder. For the rest, you can usually have at least some code implicitly written. (How much may depend on your minimum Mac OS X version; for instance, some features were only added to Interface Builder in Mac OS X 10.3). You can install the Standard Window Handler from a Nib. You can avoid calls to SetControlProperty() if you associate properties with items in a Nib window. Buttons can be attached to command IDs, implicitly calling your Carbon Event handlers for those IDs. You can even add rudimentary auto-view-resize behavior.