Significance of the Mach Kernel

 Unlike traditional systems, under Mach a process, or "task", can consist of a number of threads. While this is common in modern systems, Mach was the first system to define tasks and threads in this way. ... The existence of ports and the use of IPC is perhaps the most fundamental difference between Mach and traditional kernels.

http://en.wikipedia.org/wiki/Mach_(kernel)