Hitting the Target
Hitting something while mounted on a fast moving horse was no easy task and required extensive and constant practice. To give you an idea of how hairy this was, consider the usual target area and the different effect a lance blow would have on each.
Percentage of target area (assumming the opponent is another mounted knight with lance)
Percentage Target
26% Shield
24% Exposed body of knight
22% Horse
14% Helm (helmet)
6% Saddle
6% Arms of knight
2% Lance
The other fellow was covered in armor, so the chance of inflicting a mortal or incapacitating wound was slight. It could be done, though, and jousters were sometimes killed if their opponents blunt lance managed to hit the eye slit and deliver a fatal blow to the head. What an armored knight had to fear most were broken bones. The best way to break the bones of an opponent was to send him flying off his horse. Thus the goal of jousting and of actual combat converged. But each portion of the the enemy target had a different probability of producing a fall if hit.
The shield was usually the largest target, but it was designed to take a blow from lance or sword and cause that blow to glance off. Later in the Medieval period, special concave shields were used for jousting, which made it easier to get a lance breaking blow. This, of course, made jousting less valid as combat practice, but this was the direction jousting had been going in all along.
The next largest target was the exposed body of the knight. Actually, this target was split up into several smaller targets and the area presented by these targets constantly varied as the rider moved about in the saddle and changed the position of his shield and lance. Nevertheless, this was the best target to hit if you wanted to unhorse the other fellow, or simply do him some injury from the blow.
Hitting the horse would most likely bring the other fellow down, but probably in such a way that he would be unharmed. Sticking your lance into a heavy target like a horse might get yourself unhorsed and would most likely break the lance. Then again, don't forget the knight's affection for horses, even enemy horses. Sticking the horse was considered low class and was generally avoided, if only because it would cause you to lose points, or even get disqualified.
The helmet (or helm) was a rather elaborate affair that covered the entire head and face. It was well rounded and, while a lucky blow could kill the other fellow, the blow was more likely to slide right off to no effect.
The saddle was rather large by modern standards and securely attached to the horse. Hitting the saddle might shatter your lance and knock the horse off balance, but it was unlikely to unhorse the rider.
Hitting the arms of the other fellow might do some serious damage, but would more likely be a glancing blow with no effect.
The other fellow's lance was a small target, but it was always the goal of an eager knight to shatter it with ones own. Difficult to do and not worth going after in combat.