In medieval times knight was the name of noble horseman warrior, well-trained and equipped for battles with enemy horsemen or infantry. Knights became the most effective units on battlefields very quickly and have held this superiority for centuries, and not without reasons.
First reason of martial superiority of knights was constant training that almost every knight or would-be knight took from childhood. Would-be knights, that were called squires or sword-bearers, learned to ride, to care about martial weapons of adult knights and to fight with these weapons. That could be swords, battle axes or maces. Squires were taught to put armor on and off the knight they served. Later they started to learn themselves how to wear armor and to fight in it without considerable difficulties. Also there were trainings of strength and stamina to forge the body and the spirit of the young squire. This combined learning and servitude took all the time of young sword-bearer, effectively preparing him to his future career of a warrior. Yet usual militiamen, recruited usually from peasants, rarely had any combat experience and were poor opponents for such well-trained person.
Second reason was that, as mentioned above, every knight had set off for a battle protected by well-crafted armor. In the early Middle Ages their bodies were protected by chain mails and helmets and shields, and in late Middle Ages knights’ armor evolved to entirely metallic suits with mobile joints that protected knight’s body from blades and arrows both. Helmets and shields also evolved to grant the noble warrior more and more protection. At the same time, irregular armies gathered mainly of peasants, had little or no armor to protect themselves. At the best, average militia warrior had armor crafted of leather, and more often his armor was made of padded cloth or wood, which granted protection significantly poorer than armor made of iron or steel does. And even this small amount of protection was often negated by artificial clumsiness of the armor-wearing soldier that was not used to wear it. That is why knight with his superior quality armor and habit to fight in it has little difficulties to defeat even several worse protected opponents.
Third reason of martial superiority of knights was their superiority in weapons. These noble horsemen used swords and battle axes forged of iron or, later, of steel. Also they had their sword-bearers to care about weapon and to set it in condition suited best for battle or storage. Weapon treated such show their best characteristics in combat, and, hence, its owner has chance to damage his opponent harder than if his weapon was dull and unprepared. Also it is necessary to consider the fact that majority of average recruits in the Middle Ages were armed with weapons that were stored in their lords’ arsenals for years sometimes, and that majority of these recruits could not took care of their weapons properly, at least before they went through their first training. This also shifted balance towards the knight’s victory.
Fourth reason was the war-horse. These animals were specially trained to stay calm in the middle of the fighting, and to be useful to their owners in battle. They were taught to bite and kick other horses and infantrymen, to knock people down on the ground and to trample them down to death. Also these horses were usually equipped with specially crafted armor to protect head, neck and chest of the steed from enemy swords and spears, thus reducing the risk of death or injury for the animal. Warhorse was stronger and larger than average horses to be able to endure load of its own armor and the knight – in armor too. Size and strength and special training of warhorse helped the knight much to fight more effective and deadly than if he would ride an average untrained horse with ordinary strength and stamina.
Another reason, concerned with horses in battle, is that the horseman has higher position, and thus can hit his opponent harder from the back of his horse, than if he stood near this opponent on the ground. Gravity helps the horseman, granting additional strength to his strikes from above, while unmounted warrior trying to hit the horseman should overcome gravity striking from below, and this obviously weakens his attacks for a certain degree. And warriors without spears long enough to hit the knight off the saddle had little chance to defeat well-armed and well-protected warrior on well-protected and well-trained horse.
Also it is necessary to consider, that armor and weapons, leaving alone warhorses, their armor and grooming, were very expensive, and no peasant or townsman could allow to equip himself as knight should be equipped. Only noble land lords, collecting taxes from their vassals and merchants that moved through their lands, could spent money enough to buy and maintain in proper conditions all the equipment needed for the knight to fight. That’s why knights were not common, but rather elite troops nothing could withstand.
Of course, knights had their weaknesses too. Heavy armor used to protect the horseman hampered his movement while he was unmounted or kicked off the saddle, making the knight fighting on his feet much more clumsier opponent. Sometimes knights could not even get up to their feet after being knocked down. Another trouble with knights was that every one among them had fought on his own. It was hard to force a bunch of knights to act and fight as a squadron. They refused to coordinate their actions, striving to glory and victory for themselves, and acted as they wanted, being not a unity, but rather a group of opportunists put together.
Also majority of knights disdained ranged weapons and underestimated their possibilities. Not without a reason, though. Only in England archers trained to use english longbows could effectively pierce knights’ armor with their arrows, but this was rather an exception. Until the cross bows and, later, powder guns were invented, little ways existed to effectively oppose these well-trained and excellently equipped horsemen warriors.