The core of any operating system is it's kernel. The kernel is responsible for the most basic jobs a computer must do such as starting and stopping programs. The kernel allocates memory and disk-space and ensures that programs can run seamlessly. Darwin is the name of the OS X kernel. It is an open source project based partially on BSD source code.
You might become aware of Darwin when you start up your Terminal.app - many people think of this as 'going into Darwin', however this is not true. Darwin is allways running and without it your mac could not do anything. Darwin does not run within OS X, rather the whole of the rest of OS X runs on top of Darwin.
Since Darwin is derived from BSD's kernel, OS X is a genuine UNIX rather than a UNIX clone (not that it makes a whole lot of diference but Linux is a UNIX clone because it contains mainly original source code that was not derived from the original UNIX).
There are many components of OS X that are not Darwin (Aqua, WindowManager, Cocoa, QuickTime, etc.).
Because Darwin is such a 'regular' Unix (very much like FreeBSD, it has been fairly straightforward to bring a lot of CommandLine and graphical software to it. To run Unix graphical software, you'll need two things:
- an X Window System -- there's a free one called XFree86 that has been made available on OS X; that version is called XDarwin.
- one of the many WindowManagers available. OroborOSX is one that is being developed specifically for XDarwin.
Or, you just need one bundled pacakge with decent acceleration from http://www.apple.com/macosx/x11/
Many handy text (non graphical) applications have been ported from Linux and BSD over to OS X. You might also try a package called Fink, which provides an entire suite of GNU text and command line tools to help Linux and BSD users feel at home on their Macs.