Disclaimer - I'm no scholar, but I think I can answer your question in a simple manner (and would love to be corrected by some real historians).
The shor answer is - one is an unit, the other is a socio-political entity, that happens to be a heavy cavalrymen (and if I recall correctly - a heavy footman) in war times. A Knight was (in theory) much more than just a solder. Cataphracts while no doubt wealthy and maybe of noble birth were more of elite regiments.
Now, if you are asking about tactics and the like, I can't give you a distinctive answer. Simply put, the term Cataphract is used for a heavily armored horsemen - both lancers and archers. At the time of knights they were probably both armored and armed in a similar manner with the common analogues of weaponry and armor for each region. The exact way they were used depends heavily on the time-period.