I'd add a voice of agreement to the barrage of answers saying that the "knights" were
both military "professionals" and members of an elite social class. Two things are worth noting, however. On one hand, if we're discussing the "knights" strictly or mostly as a military force, it's usually more accurate to use the more general term "men-at-arms" since--as others have pointed out--"knightly" formations in actual wars often included men of less-than-knightly social classes. FYI, the use of "men-at-arms" to describe all and sundry kinds and soldiers is just a modern bad habit--serious scholarship on medieval warfare tends to use the term as it was historically used, which was to refer to the "knightly" role in warfare.
Another is the distinction between "knights" and "nobles." I may be getting a little pedantic, but a knight (or a squire, or a banneret) without any other titles was technically a member of the gentry, which was part of the aristocracy all right but not always considered part of the higher order of "nobility." Whether all nobles were knights or not is a pretty complicated matter--back in the early Middle Ages, for example, nobles
didn't want to be knights because it was seen to be demeaning, while in the Late Middle Ages it wasn't uncommon for every nobleman (above the mere "knightly" income bracket) to be knighted as soon as they were old enough and had enough military experience.
As you probably would have figured out by now, questions like what you've posed usually have no simple, easy answers. That's why medieval European history is a whole sub-discipline unto itself.
