Mac OS X is based on the XNU kernel, a microkernel Mach kernel with a BSD userland, which makes Mac OS X's kernel a hybrid-kernel.
Mac OS X uses a Mach kernel and FreeBSD utilities at it's core.
The Mac OS X Kernel is a hybrid based on the XNU kernel derived from the former NextStep implementations. Much of the code is open source and freely available. (See links below)
Mac OS X is built on the XNU (X is Not Unix) kernel which is a hybrid combining elements of the Mach kernel and FreeBSD.
OS X was forked from the FreeBSD 2.x/3.x branch. with kernel mods and the quartz userland. the various OS X releases have corresponding FreeBSD releases. it uses the FreeBSD mach microkernel with os9 compatibility. Darwin is the development builds of OS X versions. the FreeBSD guys backport OS X tech into their OS. OS X is opensource aside from the quartz userland. OS X also includes kernel components from the NeXT step operating system.
Mac OS X is based on UNIX.
Unix
Mac OS X use Mach kernelmore about Mach: http://en.wikipedia.org/wiki/Mach_kernel
Mac OS X PPC is a version of Apple's Mac OS X operating system that runs on Power PC (PPC) architecture. This is opposed to Mac OS X for X86 which runs on Intel-based Macs. Mac OS X 10.0 through 10.5 can run on PowerPC based macs whereas Mac OS X 10.6 Snow Leopard will only run on Intel-based Macs.
Providing the Intel based computer is manufactured by Apple then their Mac OS X software can work on it. The Mac OS X user's licensing agreement prevents Mac OS X being used on computers that are not made by Apple.
No, the Registry is a feature of Microsoft Windows only. Neither the original Mac OS or the newer Mac OS X have ever used a Registry, Mac OS X is based on Unix which has never used a Registry.
The Darwin kernel of Mac OS X is open source, as are many other parts of the OS. (See links below) but Mac OS X itself is not open source and Apple only allows it to run on Apple hardware.
Mac OS X is pronounced Mac OS Ten. It follows on from Mac OS 9 which was the last version of what is now known as the classic Mac OS and it saw the introduction of the new UNIX based OS so the Roman numeral X was adopted to mark the change.