Mac OS X
Mac OS X is a commercial UNIX like Operating System produced at Apple. Much of the kernel is based on Open Source FreeBSD. The userspace environment is very much different, and configuring certain simple things (such as NFS) can be quite a trial; other things, such as adding, say, external hard drives are quite easy. If one is interested how Mac OS X works internally it is probably wise to visit Open Darwin's site where source code is freely available as well as good documentation.
Apple Mac OS X versions and their codenames
10.0 Cheetah 10.1 Puma 10.2 Jaguar 10.3 Panther 10.4 Tiger <--------------------------- Change from powerpc macs to intel based macs. 10.5 Leopard 10.6 Snow Leopard 10.7 Lion 10.8 Mountain Lion
Programming with Mac OS X
If you want to program Carbon or iPhone app programming you would use Xcode which is proprietary for macs. Getting this requires you to get a developer account at developer.apple.com. Xcode has an iphone simulator where you can test your newly built apps with and there is a fee if you want to release your apps to the public ($99?).
Several nifty shortcuts