What is an OS kernel ? How does it differ from an operating system?

A kernel is the part of the operating system that mediates access to system resources. It's responsible for enabling multiple applications to effectively share the hardware by controlling access to CPU, memory, disk I/O, and networking.

An operating system is the kernel plus applications that enable users to get something done (i.e compiler, text editor, window manager, etc).

The technical definition of an operating system is "a platform that consists of specific set of libraries and infrastructure for applications to be built upon and interact with each other". A kernel is an operating system in that sense.

The end-user definition is usually something around "a software package that provides a desktop, shortcuts to applications, a web browser and a media player". A kernel doesn't match that definition.

So for an end-user a Linux distribution (say Ubuntu) is an Operating System while for a programmer the Linux kernel itself is a perfectly valid OS depending on what you're trying to achieve. For instance embedded systems are mostly just kernel with very small number of specialized processes running on top of them. In that case the kernel itself becomes the OS itself.

I think you can draw the line at what the majority of the applications running on top of that OS do require. If most of them require only kernel, the kernel is the OS, if most of them require X Window System running, then your OS becomes X + kernel.

It seems that the original metaphor that got us the word "kernel" for this in the first place has been forgotten. The metaphor is that an operating system is a seed. The "kernel" of the seed is the core of the operating system, providing operating system services to applications programs, which is surrounded by the "shell" of the seed that is what users see from the outside.

Some people want to tie "kernel" (and, indeed, "shell") down to be more specific than that. But in truth there's a lot of variation across operating systems. Not the least these variations is what constitutes a "shell" (which can range from Solaris' sh through Netware's Console Command Interpreter to OS/2's Workplace Shell and Windows NT's Explorer), but there's also a lot of variance from one operating system to another in what is, and isn't, a part of a "kernel" (which may or may not include disk I/O, for example).

It's best to remember that these terms are metaphors.

Further reading

• https://superuser.com/a/329479/38062