Apple's open source projects allow developers to customize and
enhance key Apple software. Through the open source model, Apple
engineers and the open source community collaborate to create
better, faster and more reliable products for our users.
Beneath the appealing, easy-to-use interface of Mac OS X is
a rock-solid foundation that is engineered for stability, reliability,
and performance. This foundation is a core operating system commonly
known as Darwin. Darwin integrates a number of technologies, most
importantly Mach 3.0, operating-system services based on 4.4BSD
(Berkeley Software Distribution), high-performance networking
facilities, and support for multiple integrated file systems.