NeXTSTEP was built on the Mach kernel (developed at Carnegie Mellon University by Richard Rashid and Avie Tevanian and derived originally from 4.3BSD to support operating system research) and the UNIX-derived BSD (Berkeley Software Distribution, BSD is a discontinued operating system based on Research Unix, developed and distributed by the Computer Systems Research Group (CSRG) at the University of California, Berkeley.

• It was initially built on 4.3BSD-Tahoe.
• It changed to 4.3BSD-Reno after the release of NeXTSTEP 3.0.
• It changed to 4.4BSD during the development of Rhapsody

Mach's derivatives are the basis of the operating system kernel in GNU Hurd and of Apple's XNU kernel used in macOS, iOS, iPadOS, tvOS, and watchOS. Richard Rashid founded the Microsoft Research division and Avie Tevanian was head of software at NeXT, then Chief Software Technology Officer at Apple.

NeXTSTEP (sometimes NeXTstep, NeXTStep, and NEXTSTEP) comprises:

• a Unix operating system based on the Mach kernel, plus source code from BSD
• Display PostScript (DPS) and a proprietary windowing engine.  DPS is a 2D graphics engine which uses the PostScript (PS) imaging model and language (originally developed for computer printing) to generate on-screen graphics. To the basic PS system, DPS adds a number of features intended to ease working with bitmapped displays and improve performance of some common tasks
• the Objective-C language and runtime a general-purpose, object-oriented programming language that adds Smalltalk-style messaging to the C programming language. In addition to being used by  NeXTSTEP it was the standard programming language used by Apple for developing macOS and iOS applications using their respective application programming interfaces (APIs), Cocoa and Cocoa Touch.
• an object-oriented (OO) application layer, including several "kits"
• development tools for the OO layers.

NeXTSTEP is a preeminent implementation of the latter three items. The toolkits are the prime development system for all of the software on the machine. It introduced the idea of the Dock (a toolbar-like application launcher) and the Shelf ( used as a repository to store links to commonly used files, directories and programs, and as a temporary "holding" place to move/copy files and directories around in the file system hierarchy).

NeXTSTEP also originated or innovated a large number of other Graphical User Inteface (GUI) concepts which became common in other operating systems: 3D "chiseled" widgets, large full-color icons, system-wide drag and drop of a wide range of objects beyond file icons, system-wide piped services, real-time scrolling and window dragging, properties dialog boxes called "inspectors", and window modification notices (such as the saved status of a file). The system was among the first general-purpose user interfaces to handle publishing color standards, transparency, sophisticated sound and music processing (through a Motorola 56000 DSP), advanced graphics primitives, internationalization, and modern typography, in a consistent manner across all applications.

A preview release of NeXTSTEP (version 0.8) was shown with the launch of the NeXTComputer on October 12, 1988. The first full release, NeXTSTEP 1.0, shipped on September 18, 1989.The last version, 3.3, was released in early 1995, by which time it ran on not only the Motorola 68000 family processors used in NeXT computers, but also on Intel x86, Sun SPARC, and HP PA-RISC-based systems. NeXTSTEP was later modified to separate the underlying operating system from the higher-level object libraries. The result was the OpenStep Application Program Interface API, which ran on multiple underlying operating systems, including NeXT's own OPENSTEP, Windows NT and Solaris. 

The NeXTSTEP versions shipped with the NeXT Computing Platforms are shown here:

Year	Model	Processor	Manufacturer	Clock Speed	MHz/GHz	Operating System
1989	NeXTComputer	68030	Motorola	25	MHz	NeXTSTEP 1.0
1990	NeXTcube	68040	Motorola	25	MHz	NeXTSTEP 2.0
1990	NeXTstation	68040	Motorola	25	MHz	NeXTSTEP 2.0
1992	NeXTcube Turbo	68040	Motorola	33	MHz	NeXTSTEP 2.2
1992	NeXTstation Turbo	68040	Motorola	33	MHz	NeXTSTEP 2.2

Our Version 3.1 NeXTSTEP is shown in the following  NeXTSTEP Version History

Version	Date	Notes
0.8	October 12,1988	NeXTStep Digital Webster, Complete Works of William Shakespeare, netboot, NFS
1	1989
2	September18, 1990	Support for the NeXTstation, NeXTcube (aka m68040 cube). Support for floppy disk, CD-ROM, Fax modems, and colorgraphics. Workspace Manager now has the Shelf, copies performed in background, black hole is replaced by recyclericon. Terminal.app. Dynamic loading of drivers.
2.1	March 25,1991	Support for the NeXTdimension board. TeX, Internationalization improvements. New machines bought with 2.1 includedLotus Improv.
2.2		Support for the NeXTstation Turbo
3	September 8,1992	Project Builder, 3D support with Interactive RenderMan, Pantone colors, PostScript Level 2, Object Linking andEmbedding, Distributed Objects, Database Kit, Phone Kit, Indexing Kit, precompiled headers, HFS, AppleTalk, and NovellNetWare.
3.1	May 25, 1993	First release for the Intel i386 architecture, introducing fat binaries. A fat binary (or multiarchitecture binary) is a computer executable program or library which has been expanded (or "fattened") with code native to multiple instruction sets which can consequently be run on multiple processor types.[1] This results in a file larger than a normal one-architecture binary file, thus the name
3.3	Feb-95	Support for the PA-RISC and SPARC architectures added, introducing Quad-fat Binaries. Last and most popular versionreleased under the name NEXTSTEP. Referred to as NEXTSTEP/m68k, NEXTSTEP/Intel, NEXTSTEP/SPARC.NEXTSTEP/PA-RISCDelivered on 2 CDs: NeXTSTEP CISC and NeXTSTEP RISC. The Developer CD includes librariesfor all architectures, so that programs can be cross-compiled on any architecture for all architectures.
4.0 beta	1996	Very different user interface. Notable as being a precursor of many ideas later introduced in the macOS Dock.Allegedly dropped due to complaints of having to re-teach users but not for technical reasons (thenew UI worked well in the beta).
4	Jul-96	Support for the PA-RISC architecture dropped. Support for m68k, i486, and SPARC architectures. Initial Release ofOpenStep for Windows.
4.1	Jan-97	Support for m68k, i486, and SPARC architectures, and OpenStep for Windows, under OPENSTEP Enterprise (NT only).
4.2 Pre-release 2	Sep-97	Pre-release 2 circulated to limited number of developers before OpenStep and Apple acquisition.
AppleRhapsody	August 31,1997 -October 27,2000	While released after the Apple merger, these versions are still very close to NeXTSTEP/OPENSTEP. Arguably closer toNeXTSTEP than to Mac OS X. For example, they can still be used as remote display via NXHost.

 

Other Software by NeXT:

Item	Manufacturer	Platform	Format	Date
Disk Caddy Containing Many NeXT Discs	Next	NeXT	3.5 Disk	Unknown
Pages by Pages 1.0 and 1.5.1	Next Step	NeXT	Disk 3.5	Unknown
Impact!	The Next Frontier Corporation	Commodore Amiga	3.5" floppy disk	1985
Auto Route The Intelligent Map	Next Base	PC	3.5 Disk	1988
Acorn CD-R Pre-Production Disc	Next	Early CD-R	CD-R	1990
CDTV Work In Progress	Next	CDTV	CD-R	1990
CD-ROM Sampler for Archimedes	Next Technology	Acorn Archimedes	CD-ROM	1990
AutoRoute Express UK & Ireland 3.04	NextBase	PC Windows 16-bit	3.5" Floppy Disk	1992
Autoroute Express	Next Base	PC	3.5 Disk	1992
NextSTEP 3.1	NeXT	NeXT	Disk 3.5	1993