Definition
The most simple way to define a knight is, that he was a man who was granted the title of knighthood by either a monarch, political or religious leader for their service in a military capacity. A knight was often a vassal who could serve as an elite bodyguard, military leader or skilled fighter, usually paid under the form of land holdings.
The path to knighthood
The path towards knighthood was long and required devotion from early childhood, there were many risks as the knight's apprentice would be exposed to many dangers during his time as a page or squire. Only Christian males could become knights, and usually only the son of a noble or wealthy person was even allowed to be taken as an apprentice, however, paging and squiring for a knight was not the only way to become a knight. Before the Late Middle Ages, it was not uncommon for a soldier or squire to earn the title of knighthood for an extraordinary display of military prowess on the battlefield and be rewarded for their service by a religious or political figure - which meant that even a commoner had the chance of becoming a knight.
Page
A page was a boy at around the age of 7, who was taught how to handle a horse, how to use a weapon(though they used wooden training weapons, rather than real steel) and how to hunt and survive outside of civilization when amidst the wilderness. They also learned how to clean and maintain the armor of the knight they were paging for, and while being around them would learn of courtly behavior and chivalry. Once they grew up more, they were exposed to the basics of warfare, swordsmanship, horseriding, and combat.
Squire
Once the page reached the age of 14, they were exposed to more training and potential to gain experience, but also had bigger responsibilities - they were allowed to own their own armor, where the page borrowed it. Squires learned how to fight with real weapons and always accompanied the knight they squired for - whether it be in war or peace, so that they may firsthand witness how a knight behaved, this was intentionally done in order for the squire to have a role model to look up to and be able to directly emulate once they become a knight in the future. Squires were required to master the "seven points of agilities"(most of the time being fully armored while mastering these skills) which consisted of - riding, swimming and diving, wrestling, fencing, jumping, climbing, tournament participation and very importantly - using all kinds of weapons that were available at the time. Squires also bore the responsibility of maintaining and looking after the knight's horses, holding additional equipment such as lances or shields and to attend to whatever other duties were required of them.
Becoming a knight
The Accolade, or more commonly known as the knighting ceremony was usually held during a great feast or Christian holiday and could've also occurred during other important celebrations - such as a nobleman's wedding. On the eve of the ceremony, a ritual bath was followed by a prayer vigil at night - and on the day of the ceremony, the knight-to-be swore an oath and then would've been dubbed on the shoulders with a sword by either a fellow knight, noble or religious figure awarding the honor, and the former squire would then rise from his knees as a knight.
Equipment
A common comparison between medieval and modern forces was that a knight on the battlefield was the modern world's equivalent of a tank, which is not a statement that is far from the truth. When it came to warfare, a knight was an elite cavalryman donned in heavy armor(chainmail or plate depending upon the time period) and were equipped with lances, arming swords or other weapons which they could utilize from horseback and do a great deal of damage to their foes. For a very long time during the Middle Ages, the cavalry dominated the battlefield due to the effectiveness of knights.
Responsibilities
A knight had to always be ready to serve their liege - whether it be a lord, bishop or a king. They had to abide by Christian virtues and conducts of honor - which depending on their devotion towards Christianity, could've prompted them to abide by a complex set of inflexible laws concerning justice and morality. Knights had to be familiar with Christian texts such as the Holy Bible(which was preserved in the Latin language) and to also know the stories of famous Saints and Christian apostles from which they could draw their faith from - which meant that a knight was required to have some Christian education and knowledge of Latin. It was inevitable that a knight would also become an aristocrat - which brought many privileges, and with them, many responsibilities. While they might've been proficient soldiers - they, as sworn vassals who most likely owned land, had to bear military, economic and political obligations to their liege as vassals.