Overview. NeXTSTEP (also stylized as NeXTstep, NeXTStep, and NEXTSTEP) is a combination of several parts: . a Unix operating system based on the Mach kernel, plus BSD; Display PostScript and a proprietary windowing engine; the Objective-C language and runtime; an object-oriented (OO) application layer, including several "kits"; development tools for the OO layers. …