[Repurposed from a reply in a Hackernews thread]
Apple looked at buying in an OS after Copland failed. But all the stuff about Carbon, Blue Box, Yellow Box, etc. -- all those were NeXT ideas after the merger. None of it was pre-planned.
So, they bought NeXTstep, a very weird UNIX with a proprietary, PostScript-based GUI and a rich programming environment with tons of rich foundation classes, all written in Objective C.
A totally different API, utterly unlike and unrelated to Classic MacOS.
Then they had to decide how to bring these things together.
NeXT already offered its OPENSTEP GUI on top of other Unixes. OPENSTEP ran on Sun Solaris and IBM AIX, and I think maybe others I've forgotten. Neither were commercial successes.
NeXT had a plan to create a compatibility environment for running NeXT apps on other OSes. The idea was to port the base ObjC classes to the native OS, and use native controls, windows, widgets etc. but to be able to develop your apps in ObjC on NeXTstep using Interface Builder.
In the end, only one such OS looked commercially viable: Windows NT. So the plan was to offer a NeXT environment on top of NT.
This is what was temporarily Yellow Box and later became Cocoa.
Blue Box was a VM running a whole copy of Classic MacOS under NeXTstep, or rather, Rhapsody. In Mac OS X 10.0, Blue Box was renamed the Classic environment and it gained the ability to mix windows with NeXT windows.
But there still needed to be a way to port apps from Classic MacOS to Mac OS X.
So what Apple did was go through the Classic MacOS API and cut it down, removing all the calls and functions that would not be safe in a pre-emptively multitasking, memory-managed environment.
The result was a safe subset of the Classic MacOS API called Carbon, which could be implemented both on Classic MacOS and on the new NeXTstep-based OS.
Now there was a transition plan:
• your old native apps will still work in a VM
• apps written to Carbon can be recompiled for OS X
• for the full experience, rewrite or write new apps using the NeXT native API, now renamed Cocoa.
• incidentally there was also a rich API for Java apps, too
Now there was a plan.
Here's how they executed it.
1. Copland was killed. A team looked at if anything could be salvaged.
2. They got to work porting NeXTstep to PowerPC
3. 2 main elements from Copland were extracted:
• The Appearance Manager, a theming engine allowing skins for Classic MacOS: https://en.wikipedia.org/wiki/Appearance_Manager
• A new improved Finder
The new PowerPC-native Finder had some very nice features, many never replicated in OS X... like dockable "drawers" -- drag a folder to a screen edge and it vanished, leaving just a tab, which opens a pop-out draw. Multithreading: start a copy or move and then carry on doing other things.
The Appearance Manager was grafted onto NeXTstep, leading to Rhapsody, which became Mac OS X Server: basically NeXTstep on PowerPC with a Classic MacOS skin, so a single menu bar at the top, desktop icons, Apple fonts and things -- but still using the NeXT "Miller columns" Workspace file manager and so on.
Apple next released MacOS 8, with the new Appearance control panel and single skin, called Platinum: a marginally-updated classic look and feel. There were never any official others, but some leaked, and a 3rd party tool called Kaleidoscope offered many more.
So some improvements, enough to make it a compelling upgrade...
And also to kill off the MacOS licensing programme, which only covered MacOS 7. (Because originally 7 had been planned to be replaced with Copland, the real MacOS 8.)
MacOS 8 was also the original OS of the first iMac.
Then came MacOS 8.1, which also got HFS+, a new, more efficient filesystem for larger multi-gigabyte hard disks. It couldn't boot off it, though (IIRC).
MacOS 8.1 was the last release for 680x0 hardware and needed a 68040 Mac.
Then came the first PowerPC-only version, MacOS 8.5, which brought in booting from HFS+. Then MacOS 8.6, a bugfix release, mainly.
Then MacOS 9, with better-integrated WWW access and some other quite nice features... but all really stalling for time while they worked on what would become Mac OS X.
The paid releases were 8.0, 8.5 and 9. 8.1, 8.6, 9.1 and 9.2 were all free updates.
In a way they were just trickling out new features, while working on adapting NeXTstep:
1. Rhapsody (Developer Release 1997, DR2 1998)
2. Mac OS X Server (1.0 1999, 1.2 2000)
3. Mac OS X Public Beta (2000)
But all of these releases supported Carbon and could run Carbon apps, and PowerPC-native Carbon apps would run natively under OS X without the need for the Classic environment.
Finally in 2001, Mac OS X 10.0 "Cheetah".