The GUI layer, AppKit, provides all you would expect from a GUI toolkit—classes for creating windows, widgets, etc., as well as an event loop for managing user interaction.
Nothing calling itself an "application kit" would be complete without a simple mechanism for creating document-driven applications. OpenStep’s NSDocument and related classes make it very easy to create such applications by overriding a small number of methods.
Since OpenStep was built around a PostScript display model, the concept of WYSIWYG is built into the toolkit. Anything that can be displayed can also be printed.