Controlling VFDs from your PC?

Hi again!

Could someone give me a quick rundown on how spindle speeds are
controlled from your PC via VFD or Spindle drive?
What are the components and what sort of signals are used
(step/direction?).
I'm getting ahead of myself in this Bridgeport conversion, but down
the road it would be nice to have the spindle motor under PC control.
In the meantime, I'd like to start researching.

Many Thanks
Sean