Structure of Mac OS X

Mac OS X (10.0 through 10.3, and soon 10.4) is a completely different operating system from Mac OS Classic.

Preemptive multitasking with memory protection
True pervasive multi-user capabilities
Derived from BSD UNIX
Mostly compatible with popular UNIXes such as BSD and Linux
Parts are open-source

Mac OS X releases are often referred to by codename: 10.0 is Cheetah, 10.1 is Puma, 10.2 is Jaguar, 10.3 is Panther, and 10.4, when it comes out, will be Tiger.