A short answer to the best of my ability:
In feudal society the knights were the landed nobles whom the lord or king supported in exchange for undying loyalty and military service in war. They were the rich land-owners who lorded over the serfs and kept the country working at the fundamental level. In battle, they could afford the best equipment: armour, weapons, and a horse (in Medieval Europe the upper class always fought on horseback), and they were hailed generally as the most important soldiers on the field (even when new technologies were rendering them obsolete). In war they were eventually phased out of the picture by the increasing efficiency of gunpowder weapons. In European society landed gentry remained a tangible concept until at least the early 20th century, when immense social upheavals (in some cases linked to the World Wars) finished it off for good. In Great Britain today (one of the only kingdoms still left over from the Medieval period) there are still knights, but they are given the title for reasons other than hereditary control over land.
This is obviously a very, very short answer to your question, but it should give you some idea.
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