The modern Mac operating system is termed OS X, and its many variants (10.1, 10.2, and the current 10.4) have been given feline code names - Cheetah, Jaguar, Panther, Tiger, etc. The next one (10.5) is already codenamed Leopard.
It is essentially a current-day version of NeXTSTEP with the Aqua interface wrapped around it - that is to say, a BSD-like UNIX system running on top of the Mach microkernel.
The best currently available online reference to its internals is Amit Singh's What Is Mac OS X?, which I recommend heartily to anyone coming from mainstream UNIX systems like Linux and BSD - it explains most of the rationale and heritage of Mac OS X in a exceptionally clear and well written fashion, and draws appropriate comparisons as it goes along.
Amit has also recently published a must-have book on Mac OS X, and made available a vastly extended version of its initial chapter covering Mac OS X's precursors and history.
O'Reilly also has a great book for people coming to Mac OS X from other UNIXes called Mac OS X for Unix Geeks:
Tools:
Networking:
- GSM and GPRS scripts for T68i
- GSM/GPRS Dial-up
Essentials:
Unix:
- Fink
- sendmail HOW-TO (pre-Panther)
- Mac OS X Ports - not as interesting as Fink, but might be useful...
- DarwinPorts
- Marc Liyanage - Ports
- Common Problems and Troubleshooting
Carbon/Cocoa:
Mach and Mach-O
- How Mac OS X executes applications - on otool and .dylibs.
Neat Stuff:
- Menu Items applications that use menu items.
- PNG Icons
- Tomcat and Cocoon (Tomcat 3.2.1 and Cocoon 1.8.2, as an installer package)
- Java and Tomcat on Mac OS X
- tun/tap driver, useful for running QEMU
- Mac OS Through The Years - turns out I remember pre-6.0 versions, even though I spent many years running 6.0.4 and 7.1...