What's the kernel?

Here's the de rigeur definition of Kernel:

The smallest, most basic piece of an object

I work from a home office, and lack the usual opportunities to gab around the water cooler and post on internal lists, so this is my soapbox.

I'm a Drupal developer, so a lot of this will be about projects I'm involved in, but it'll also be about the rest of the world that's pretty interesting as well.