Based on the Mach 3.0 kernel, which Apple acquired when it bought NeXT in 1997, Darwin also bears a few other remnants of its NeXT heritage, such as its object-oriented C++ development environment. What it does not include is the Mac OS X user interface, known as Aqua, nor any of the higher-level development environments, such as Carbon and Cocoa, that are central to Mac OS X's functionality. Darwin will run on some Intel hardware, although you have to download the source code and compile it yourself.
You get a very stable and surprisingly mature Unix environment. That said, hardware support is un-fortunately limited. FireWire is not supported, nor are SCSI cards from ATTO and Adaptec.
If you have one of the systems that Darwin does support (they are listed on the Web site), you can have a free, industrial-strength Unix environment that runs like the clappers on fast Mac hardware. Standard Unix apps are included, such as the emacs text editor and Apache Web server. Other Internet services are under development by third parties.
For anyone looking for a preview of Mac OS X, ignore Darwin and wait a couple of months.