Where is minimum focus distance measured from?
It's measured from the focal plane. On a film camera that's where the film sits; on a digital camera it's where the optical center of the sensor stack sits. On almost all Nikon bodies, this point is marked with a small symbol (circle with a line through it).
Where is working focus distance measured from?
Working focus distance is a term used with macro lenses. This is the distance from subject to the front of the lens at maximum magnification (remember also that some macro lenses extend outward as they focus closer). With insects and other living things, usually you want longer working distances rather than short ones. Shorter focal length lenses (60mm macro) tend to have shorter working focus distances than longer focal length lenses (105mm macro). Light also has to get to your subject past the lens. With very short working distances sometimes you can't get light into the subject. For example, Nikon's current macro flash setup works sufficiently for the 105mm and 200mm lenses at minimum working distance, but on the 60mm the angle of the light to the subject is very severe.
