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

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