macOS (formerly Mac OS X)

The modern Mac operating system is now termed macOS, after an entire generation of OS X and its many variants (10.1, 10.2, and the current 10.4) having initially been given feline code names - Cheetah, Jaguar, Panther, Tiger, Leopard, these days turned into grander monikers like El Capitan, Sierra, etc.

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:

• MacJanitor, MacSniffer, Pseudo, xFiles
• Mac OS X Labs

• GSM and GPRS scripts for T68i
• GSM/GPRS Dial-up

• Boot Keys
• My Hints For Newcomers To The Mac

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:

• OmniWeb
• Fugu (SSH/sftp)
• HenWen (snort front-end)
• NetNewsWire (RSS browser)

Mach and Mach-O

• How Mac OS X executes applications - on otool and .dylibs.

• Menu Items applications that use menu items.
• 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...