AlistairSam,
Have you tried the ASCOM web site ? - it has all the information, followed by the author himself.
Try here for the overview and then follow the links for each type of device :
http://ascom-standards.org/About/HowWorks.htm
Individual standards are here :
http://ascom-standards.org/Standards/Index.htm
The ASCOM command set is specific to each type of device controller.
eg is it a focusser, rotator, camera, filter wheel, dome, telescope driver (mount) etc
New types of devices need to go though an approval process as does each new ASCOM compliant component.
New properties and methods for existing devices need the ASCOM seal of approval before being incorporated into the standard and being able to be implemented by the ASCOM driver itself.
Good luck with it all
Rally