Mach is part of the underlying technology of Mac OS X. It is a UNIX technology developed at Carnegie-Mellon University in the late 1980s and 1990s. It is a robust, "open source" operating system, which is used to provide memory and task implementation services. Darwin uses Mach 3.0. Mach provides memory management, memory protection, process scheduling and interprocess communication services.
Mach, together with BSD, forms the heart of Mac OS X, known as Darwin.
http://developer.apple.com/unix/index.html