A knight is a member of a warrior tradition that is heavily interwoven in the feudal culture of the Seven Kingdoms and the Faith of the Seven. Knights occupy a social standing between that of lords and smallfolk. Contrary to the nobility, this rank is not hereditary and it is possible for the baseborn to become knights. Knights are referred to with the title "Ser".
Knighthood has its roots in Andal culture and was brought to Westeros during the Andal Invasion. The military success of the Andals' mounted knights and their steel armament proved vital to the Andals' conquest of Westeros. The influence of the Andals on the culture of Westeros has made knights the backbone of warfare in the Seven Kingdoms. However, knighthood holds less cultural significance in areas of Westeros that are less assimilated to Andal culture, such as the North, the Iron Islands, and Dorne.
Knights are supposed to be brave, courageous, honorable, true to their word, and loyal to their feudal overlord, and to defend their faith. In practice, most knights fall short of sustaining such high and noble ideals. Many don't even try particularly hard, and seek knighthood mainly for the prestige and opportunities.
To a degree, that is understandable and perhaps unavoidable; for bastards and smallfolk, knighthood is one of the very few paths for improving their social standing and monetary prospects that does not involve seriously restrictive vows such as celibacy or abstaining from marrying, having children or owning lands of their own. On the downside, knighthood by itself brings few advantages and significant expenses and risks. But it opens the doors for significant opportunities as well.
In reality, skill at arms is the most important aspect of knighthood. Knights are expected to fight whenever there is a need for them. They traditionally fight as heavy cavalry, wearing plate armor and fighting from horseback with lance, sword and shield. Some knights choose to fight with other weapons such as axes, hammers, and spears as well. They own at least one horse, and typically own two: a fierce warhorse and a milder horse for daily riding. According to Ser Jaime Lannister three hundred dragons is a fair ransom for a knight. 
Knights display a coat-of-arms on their shield and surcoat to identify themselves. They generally wear their family arms. Those who do not have family arms, or do not wish to use them, can create their own. These personal arms often portray the manner in which the man achieved knighthood or have some other personal significance. Many knights wear large crests on their helms that embellish the theme of their arms. Wealthy knights often seek to further distinguish themselves by wearing ornate weapons and armor. Lacquered or jeweled armor and capes of exotic materials are some ways that knights can flaunt their status.
Becoming a knight
The traditional process to becoming a knight has three stages.
- Page, A boy that becomes a page is attached to a knight, who becomes the boy's master. The sons of many knights and lords are sent to foster with relatives or allies, while other pages serve their own fathers. The page performs simple errands for the knight, who in turn begins to train the boy in vital skills, such as jousting and swordsmanship. Typical training involves sparring with blunted weapons and tilting at rings.
- Squire, When a boy reaches adolescence, he graduates to being a squire. Squires learn to properly care for and use of weapons, armor, and horses as well as learn about Chivalry. In time of war Squires join their masters in war, assisting them with their equipment and fighting by their side in battles. Some squires choose to never become a full knight, and live the rest of their lives as squires. This may be because the individual does not have the inclination to live a knight's martial lifestyle, or does not have the funds to properly equip himself.
- Knighthood, Any knight can proclaim another man a knight for whatever reason he chooses. This usually happens when a squire reaches adulthood and his master judges him worthy of accepting the responsibilities of a knight. A man who has not been raised in the knightly tradition can also be made a knight as a reward for service. This is often granted to soldiers or other smallfolk who have shown bravery or performed a great feat. Knighthood is considered valuable to smallfolk, as it raises a commoner's social standing. Knighthood is seen as primarily a martial position, so even the sons of powerful lords are not necessarily knighted if they are incapable of fulfilling the requirements. Doing otherwise would lose honor rather than gain it, and would make a lord and his family be held up to ridicule, this social pressure generally prevents knights from giving out knighthoods for petty or selfish reasons.
The Knighting Ceremony
The ceremony to create a knight can be simple or complex, however, it always involves the man kneeling before a knight and being tapped on the shoulders with a sword. Ceremonies usually have religious overtones involving the Faith of the Seven. When knighted, men are often charged in the name of the Seven to be just and honorable. More elaborate knighthood ceremonies involve the prospective knight sitting a night's vigil at a sept, before the figure of the Warrior, he may lay his sword before or upon the figure, and their armour piled at its base. Followed by walking barefoot from the sept to the knighting place to prove their humble hearts. They wear shifts of undyed wool to receive their knighthood, which is marked by the putting on of the swordbelt after dubbing. Newly-knighted men are also sometimes anointed with seven oils by a septon. It is considered a great honor for the recipient when individuals of high status or fame perform the ceremony.
Part of the knighting ceremony, starts with person name and House(if he has one), then:
a touch on the right shoulder with the blade. "In the name of the Warrior I charge you to be brave."
The sword moves from right shoulder to left. "In the name of the Father I charge you to be just."
Right shoulder. "In the name of the Mother I charge you to defend the young and innocent."
The left. "In the name of the Maid I charge you to protect all women....
Any knight can make a knight. 
Types of knights
- Hedge knight, A hedge knight is a wandering knight without a master. Hedge knights are so named because most of their worldly wealth is in their arms and horses and they generally must sleep outdoors, under a hedge. Most hedge knights travel in search of employment and often attend jousts to make money and display their prowess in hopes of being hired. Less scrupulous hedge knights put their martial training to use by resorting to banditry. For this reason, hedge knights are often mistrusted and considered disreputable. The term "hedge knight" itself is considered disparaging. 
- Sworn sword, Some landless knights become sworn swords to other men, taking them as their master. They act as retainers for their master, taking food, shelter, and money in exchange for their services in war, should they be needed. During a campaign, lords often hire a large number of sworn swords on a temporary basis. After the campaign ends and the lord has no further need for so many knights, they are released and become hedge knights once again.
- Landed knight, A landed knight is a knight that takes residence in a keep with accompanying land. They have their own peasants and men-at-arms, and may even take sworn swords. Landed knights are sworn to fight for the lord who holds dominion over their land. While the wealthiest knights manage more land than the poorest lords, landed knights do not have the authority to deliver justice in their land. Rather, they must appeal to their liege lord.
- Northern cavalry, Because few Northerners worship the Seven, they rarely choose to become knights. However, the warrior tradition of the North is very similar to that of knighthood. Heavy cavalrymen in the North serve a nearly identical function to knights in the South, and are considered knights in all but name.
- True Knight, A knight who embodies all that a knight should be, that is, a perfect knight exemplifying all the qualities of what knighthood stands for and fully follows oath of knighthood.
What do you think a knight is for, girl? You think it’s all taking favours from ladies and looking fine in gold plate? Knights are for killing. 
Without honor, a knight is no more than a common killer. It is better to die with honor than to live without it.
- Ser Barristan Selmy
Knights die in battle.
A knight's a sword with a horse. The rest, the vows and the sacred oils and the lady’s favors, they’re silk ribbons tied round the sword. Maybe the sword’s prettier with ribbons hanging off it, but it will kill you just as dead.
Some knights are dark and full of terrors. 
I am a knight...I shall die a knight. 
- Ser Barristan Selmy
- The Widow of the Waterfront, being scornful
... however strong or fast or skilled a knight may be, there are others who can match him.
References and notes
- A Storm of Swords, Chapter 44, Jaime VI.
- So Speak MartinSome Info About Knighthood, July 30, 1999
- A Feast for Crows, Chapter 8, Jaime I.
- A Clash of Kings, Chapter 65, Sansa VIII.
- The Hedge Knight p.472-473
- The Hedge Knight: 518
- A Storm of Swords, Chapter 34, Arya VI.
- The Hedge Knight p.458
- The Hedge Knight p.467
- So Speak Martinland owners and marriage in westros, December 19, 1999
- A Clash of Kings, Chapter 52, Sansa IV.
- A Feast for Crows, Chapter 42, Brienne VIII.
- A Game of Thrones, Chapter 57, Sansa V.
- A Dance with Dragons, Chapter 23, Daenerys IV.
- A Dance with Dragons, Chapter 27, Tyrion VII.