In reply to Ed Boyter:
Probably the simplest analogy is to say that "Ape Index" is a fairly crude way of estimating how much of a persons overall reach is due to the length of their arms and not the length of their legs.
Another way to put it (in terms of relevance to climbers) is to say that if two persons have exactly the same overall reach but the first person achieves this by having longer arms but shorter legs than the second person, then the first person will tend to be at an advantage in climbing terms (although not always and in all situations eg; long legs are often an advantage when bridging but a disadvantage when moving around overhangs or doing rockovers).
Also note that it's impossible to use this as an exact measure either since other factors affect a persons height as well, (particularly spine length) and normal anatomical variances such as whether a person has 4, 5 (normal) or 6 lumbar vertebrae can make significant differences to the height of two persons with exactly the same leg length!!
"Biggest difference in personal terms between two climbers and their relative abilities; usually technique!!" Anon.