I'm almost surprised at how many folks have never heard of NeXT and how many (particularly Mac fans) are oblivious to the exsistance of NeXTSTEP roots in OS X. To us old-timers, logging into a modern Mac is just like being right there back in 1991 on a NeXT machine.
Here's a nicely written, brief history of the evolution of Apple, NeXT, and OS X and how they all tie together. One minor warning: This is really a commercial for a book. Since I was a witness (as a user) to all of this, it's good to see it explained reasonably well. This is just an overview, not a deep exploration of history.