- "The kernel is the fundamental part of an operating system. It is responsible for providing secure multiplexing and arbitration of a machine's hardware, which it provides in the form of a set of abstractions."
- "Some operating systems, such as Linux, have Monolithic Kernels which provide rich and powerful abstractions. Others, such as GNU/Hurd have microkernels which provide a small set of simple abstractions. Yet others have exokernels which provide no abstractions. In the latter cases, the remaining features require small modules which can be flexibly configured."