The X Keyboard Extension essentially replaces the core protocol
definition of keyboard. The extension makes possible to clearly and
explicitly specify most aspects of keyboard behaviour on per-key basis
and to more closely track the logical and physical state of the
keyboard. It also includes a number of keyboard controls designed to
make keyboards more accessible to people with physical impairments.

There are five types of components in the server database corresponing
to five xkb symbolic names: symbols, geometry, keycodes, compat and
types which determine the keyboard behaviour. These five components
can combined together into a resulting keyboard mapping using the
'rules' component.

More information, including the complete specification, can be found
on http://www.x.org/wiki/XKB