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Mick
The issue is the difference between "tracking" and "guiding". Although similar, there are differences. Tracking simply means that to scope will automatically move to compensate for the earths rotation so an object stays in the field of view. For visual use this is all that is needed.
Even the best made mounts will experience some error in their tracking however, which is where guiding comes in. Guiding usually involves a second camera running thought another scope mounted with the main scope ( or operating on an "off axis guider" which uses a part of the field of the main scope) to track a star near the objects. It sends adjustments to the mount so that the tracking is made more accurate. This is helpful with long exposure images to ensure that the images are as accurate as possible. It is not usually needed for visual use.
PHD is a program that operates with a guide camera to interface between the camera and the mount.
Hope the above is clear enough!!
Malcolm
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