Knights are a class exclusive to Lawful Good orthodox followers of Titanius, and while they are not fully exclusive to Humans or nobility, the majority of Knights do belong to those two groups, for several reasons.
The idea of Knighthood is fundamentally a Human thing. It originates from the specifically feudal structure of the Kingdom of Etrand, which itself in many ways originated as a compromise between the adminsitration model imported from Froturn and the pre-existing tribal ways. Froturn with it's professional mercenary legion just has a different structure - Knighthood is an alien concept to High Elves and non-Humans in general - but not the Half-Elves of Artaburro.
Normally, knights come from Etrandish, Etrancoasti or Artaburran nobility, but not always. Very often, the knighthood adopts orphan children and trains them to be knights - this is how non-noble Humans, especially Half-Elves (other than the ones from Artaburro), and also Half-Orcs, and even the various non-Human minorities of Etrand manage to make up a small percentage of the knights.
Knights who stray away from the Path of Light will usually become Death Knights.
The origins of the Knights and Paladins can be traced all the way back to the Knights of the Golden Light, in first century Etrand, but many believe that their origins go back even further, some considering the magic-using bodyguards of Queen Salvia of Froturn to be the first real knights, rather than battlemages. In fact, the Book of Visions contains several magic-using elven warriors who - while most likely fictional and mythical rather than real - can be thought of as proto-Paladins of a sort.
The earliest knightly order, the Knights of the Golden Light was in fact a religious order - a monastic order consisting of warrior-monks who happened to use swords, wear heavy armour and prefer fighting from horseback - they still had to take vows of celibacy and attend to various other monkly duties, identifying themselves as the "Monks of War" and "Priests of War". In 102 AEKE, the majority of the memberes of said order were massacred during the infamous Knights' Massacre, and the remaining survivors decided not to recreate the order, instead preferring to follow a format where each master took up one apprentice, passing the knowledge down from generation to generation.
It was during the Demonic Invasions of Artograch, that the second knightly order was founded: the Knights of the Blood Red Light. They completely rebranded what it meant to be a knight, dropping the celibacy and various other monkly aspects, starting to identify as "Soldiers of the Church" and "Soldiers of the Faith". Knights were allowed to marry and own land (albeit land-owning knights were forced to donate certain amounts of their income to the order, and still attend their orderly duties). These decisions made it attractive for noble families to give their excess sons to the order - soon, several new knightly orders were founded, trying to jump on the bandwagon and capitalize on the popularity of the order that de facto started it all.
For the following centuries, the looming threat of the undead and the demons kept the various knightly orders popular and well-funded - in times of war, such as the various Pagan Rebellions in Hulra, these knights were called in to cut down the rebellious heathens, while during peace time, these knights would serve as a sort of peace-time police force, keeping the maurdering Orcs at bay on the southern borders, assisting the secular and religious authorities alike at fighting crime, exploring dungeons, and various other duties associated with being the military arm of the Church during peace time.
The massive scandal that rocked the Knights of the Blood Red Light during the early 6th century did not really take the other knightly orders with itself - at least, not immediately. Instead, knighthood would decline slowly after the turn of the century, with decadent noblemen being unwilling to take up this most noble profession. Additionally, the rise of de facto proto-secularism and decline of religious fanaticism all contributed to the decline of knightly orders. In fact, King Bryant I of Etrand himself proclaimed that "the age of the paladin is over, the age of religious fanaticism is over, this is the age of realpolitik" - but boy, was he wrong. The remainder of the century in which he died had events that lead to the revival and renaissane of the Knight, with recruitment getting into full swing in the 810's and 820's. In an essence, the contemporary state of affairs is reminisscent of the aftermath of the Demonic Invasions, when wide awareness of a perceived demonic or undead threat - no matter how unlikely - is making people very afraid, and very willing to inject money into maintaining the various knightly orders and keeping its members employed.
Appearences in media
Being a Knight comes with the bonuses of receiving all armour skills, an advanced skill at swordsmanship and blunt weapons, an advanced skill at polearms.
In the video game adaptations of the FRPG, Knights are a compromise between Warriors and Clerics, in the same way Battlemages are between Warriors and Magicians. They have higher amount of hitpoints than Inquisitors, they can use all non-ranged weapons, but have lower amount of spellpoints, they are also limited to chainmail armour at strongest.