what is 'Follow Focus'?
 

I`ve seen it discussed a few times but can`t recall where. Is there a special focusing tool for cameras?

The follow focus is a drive that allows you to operate the focus ring on a lens without directly touching the lens itself. They can be mechancial or motor driven, the latter having the option of either being hard wired or radio controlled.

You can find some follow focus controls here under mechancal accessories : http://www.arri.com/entry/camera.htm

There are other manufacturers.