Calf-kneed is a term used to describe a horse who's knee comes behind the imaginary plumbline drawn from the center of the shoulder to the ground. This horse is not an extreme example, by any means, but a few of the photos are showing the fault to a minor degree. (this is why there is so much stress about getting good conformation photos).
Calf-kneed means there is a slight-to-severe backward bend to the knee, rather than being straight (ideal) or bent at rest in the opposite direction (over at the knee / buck-kneed). It's also the most inhibiting of the knee conformation flaws, predisposing the horse to traumatic soft tissue injury due to over-extension.
This is the image I always use to illustrate. The horse on the left has severe calf knees, the middle horse has correct knees, and the one on the right is over at the knee.
I have always wanted an Impressive bred (N/N) cremello stallion that was well put together and great minded. And we found him earlier this year. He has produced some very handsome foals. Yes we stand him to the public BUT we are very selective in who we breed him to. We only breed him to 5 or 6 outside mares per year and they must be registered, well put together, etc. We currently have one mare in foal to him for a 2012 baby that we plan on keeping.