Mach is a first-generation microkernel.

Mach's basic abstractions include virtual address spaces in the form of tasks, execution contexts in the form of threads, IPC, capabilities in the form of ports, and memory objects, which enable Mach's external pager mechanism.

Controlling tasks, their virtual address space, threads, and other system objects in Mach is implemented by using ports, as opposed to other kernels' system call interface: almost all of the Mach API is implemented by sending messages to ports. Device drivers that reside in kernel space are controlled by ports, too.

Mach's ?API is well-documented.

In particular the [mach_kernel_principles] book further elaborates on Mach's concepts and principles.