In depth: My trusty Oxford Dictionary defines a kernel as "a softer, usually edible part of a nut" but offers as a second meaning: "The central or most important part of something." (Incidentally, it's this first definition that gives rise to the contrasting name 'shell', meaning, in Linux-speak, a command interpreter.) In case you're a bit hazy on what a kernel actually does, we'll start with a bit of theory.

The kernel is a piece of software that, roughly speaking, provides a layer between the hardware and the application programs running on a computer. In a strict, computer-science sense, the term 'Linux' refers only to the kernel - the bit that Linus Torvalds wrote in the early 90s.

All the other pieces you find in a Linux distribution - the Bash shell, the KDE window manager, web browsers, the X server, Tux Racer and everything else - are just applications that happen to run on Linux and are emphatically not part of the operating system itself. To give some sense of scale, a fresh installation of RHEL5 occupies about 2.5GB of disk space (depending, obviously, on what you choose to include). Of this, the kernel, including all of its modules, occupies 47MB, or about 2%.

Inside the kernel

But what does the kernel actually do? The diagram below shows the big picture. The kernel makes its services available to the application programs that run on it through a large collection of entry points, known technically as system calls.

The kernel uses system calls such as 'read' and 'write' to provide an abstraction of your hardware.

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