I've seen "Knight" being mentioned a few times and I would like to know what exactly makes a Knight, well a Knight?
Historically, a Knight had the following aspects:
First and foremost, they were Cavalry - whether lightly armored or heavily armored, but shield use as standard - and trained in both the charge with lance (long spear) and melee combat from horseback (cruciform arming sword, falchion, mace, etc.). Cavalry is the etymological root of Chevalier/Cavalier.
Second, they
served a liege lord - whether a baron, king, or church. Even hedge knights, when actually being a knight, served
somebody - even if it was only temporary and mostly for the money, and very similar to the Japanese concept of Ronin. The etymology of Knight is
servant/retainer.
Nobility was not a prerequisite for being a Knight. It made it easier, because one could better afford what a Knight required (horse, care of said horse, armor, weapons), but this stuff could be provided by a liege if one had the necessary training and skills - and could prove one had the necessary training and skills (a display of abilities, vouched for by another such as "he and I squired together for Baron Smith", etc).
I thought Knights were basically a status and they were identified by their wearing of full plate armour. I've never known Knights to have any sort of ability. I also thought the Paladin was supposed to be the typical heroic knight in shining armour.
In the late medieval period, for the most part you'd be correct, but the medieval period encompassed almost a thousand years of history. The only constants throughout the entire period are what I listed above. Other aspects that were only important during certain parts of the medieval period were:
Chivalry - didn't become a thing until late 12th century/early 13th century - so only for about the last 200 to 300 years of the medieval period.
Plate Armor - didn't really come into being until the 13th century, and didn't hit its peak until almost the 15th century - so only for about the last 100 to 200 years of the medieval period and extending into the renaissance period for another hundred or so.
Finally, the thing that most symbolized a Knight wasn't armor, or even a sword (a sword symbolized
money, and didn't necessarily signify nobility). In fact, not all knights used or preferred a sword. They often used a mace, Morningstar, or even an axe as a primary weapon; and if they did use a sword, it was just as likely (or possibly
more likely) that they used a falchion rather than a cruciform "Knightly" sword.
What most symbolized a Knight was their
spurs. Becoming a knight meant
"earning your spurs." This goes back to the stipulation that Knights are first and foremost, Cavalry.
What would a current 5th edition Knight class look like and what would be it's flavour/thing?
From a fantasy/D&D perspective, and consistent with historical context, a Knight class would have to have the following attributes/equipment:
Mounted Combat Proficiency
Melee Combat Proficiency
Lance/Long Spear proficiency and Ability to Charge/Joust
Simple and Martial Weapons
All Armor and Shields
Not a Ranged Weapon Combatant
Possible Adherence to Chivalry, Definite Fealty to a Liege
A Liege Lord or Hedge Knight status
Own a horse or have a horse provided by a Liege
Own weapons and armor or provided by a Liege
Spurs