John, a guide scope is a second smallish scope, of about 500mm focal length to which one attaches a web cam or similar type of camera.
This scope usually attaches to the main scope, and as the image is seen through the guide scope with the attached web cam or similar the information needed is transfered to the mount, so that corrections in following the object can be made.
I hope this helps a bit, some others here may be able to explain it better.
So, what it does is "guide" the mount ie "lock" onto a star captured by the guidescope cam.
A seperate scope is required for this because it exposes at a rate of 2 secs or so . The main imaging cam exposes up to 20mins or more per frame,so cant provide fast enough correcting (guiding) info to the mount for it to stay locked on the guide star continuosly.
A guide scope is any scope whether smaller of larger than the primary scope which is used to keep the target centred in the primaries point of focus so that it may be captured perfectly without distortion due to tracking errors etc
