We all know the stereotypical depiction of knights, swearing their loyalty to a lord, and slaying a dragon while wearing stylish plate-armour. Lately, I've come to the conclusion that dragons do not exist, so that made me wonder what their actual role in medieval society was. Were they just noble titles? were they military commanders (the leader of a group of infantrymen for example), or maybe a special unit like cavalry?

I'm asking specifically about knights who were properly knighted and who swore loyalty to a lord, not just a heavily armored soldier.

I understand that the middle ages was a very long period of time, and that depending on the time and place things could be very different, so for the purpose of this question we're talking specifically Northwestern Europe, during the time were you would find your stereotypical knight, around the early- to middle- middle ages, so no renaissance Italy, and before the resurgence of the big cities in general.