UNIX Architecture

The following article comes from this book:
Advanced Programming in the Unix Second Edition By W. Richard Stevens, Stephen A. Rago

In a strict sense, an operating system can be defined as the software that controls the hardware resources of the computer and provides an environment under which programs can run. Generally, we call this software the kernel, since it is relatively small and resides at the core of the environment. Following figure shows a diagram of the UNIX System architecture.

The interface to the kernel is a layer of software called the system calls (the shaded portion in Figure ). Libraries of common functions are built on top of the system call interface, but applications are free to use both. The shell is a special application that provides an interface for running other applications.
In a broad sense, an operating system is the kernel and all the other software that makes a computer useful and gives the computer its personality. This other software includes system utilities, applications, shells, libraries of common functions, and so on.

For example, Linux is the kernel used by the GNU operating system. Some people refer to this as the GNU/Linux operating system, but it is more commonly referred to as simply Linux. Although this usage may not be correct in a strict sense, it is understandable, given the dual meaning of the phrase operating system. (It also has the advantage of being more succinct.)