Knight is a term to refer to a warrior or nobleman in former times, or today to refer to a person who has been given a royal recognition. The female form of the latter is usually Dame.
Although the roots of the word knight are connected to the Old English \cniht, meaning pageboy, or simply boy, or GermanKnecht, or servant, the ideas of knighthood are arguably more closely tied to the Romanequites (see esquire). While a knight was the servant of the Crown and of God he was also very often a wealthy junior nobleman.
During the middle ages, the term knight referred to a mounted and armoured soldier. Originally, knights were warriors on horse-back, but the title became increasingly connected to nobility and social status, most likely because of the cost of equipping oneself in the cavalry. Knighthood eventually became a formal title bestowed on those noblemen trained for active war duty.
In theory, knighthood could be bestowed on a man by any knight, but it was generally considered honorable to be dubbed knight by the hand of a monarch or royalty. By about the late 13th century, partly in conjunction with the focus on courtly behavior, a code of conduct and uniformity of dress for knights began to evolve. Knights were eligible to wear a white belt and golden spurs as signs of their status. Moreover, knights were also required to swear allegiance — either to a liege lord or to a Military order.
A knight was to follow a strict set of rules of conduct. These were the Knightly Virtues. (Original knights had few of these qualities. When the church deemed knights too bloodthirsty and unruly, they intervened and began stressing the importance of virtues until the church became an integrated part of knighthood and chivalry.) The virtues included:
Mercy (Towards the poor and oppressed. They were supposed to be harsh with evil-doers.)
Fear of God
Utmost graciousness and courtesy to ladies
These virtues became more idealized as time went on. Changes in military tactics, such as the successful use of the longbow against the French cavalry in the battles of Crécy and Agincourt lessened the importance of the cavalry. (However, the true end of the knight was brought about by the use of gunpowder and guns.) In times of peace throughout the later Middle Ages and as late as the end of the 16th century, the role of the knight was promoted and extolled through highly stylized tournaments that bore little resemblance to the bloody warfare in which the "typical knight" had once participated. (Early tournaments were actually very similar to war. They originally included many participants battling each other at once in a chaotic mock war, though they later evolved to the popular, one-on-one jousting we all know.)
When even the tournaments went out of fashion, knighthood became less and less tied to warfare, and increasingly indicated social status.
Presumably there are other monarchies that also follow the practice. Modern knighthoods are typically awarded in recognition for services rendered to society, services which are no longer necessarily martial in nature. The musician Elton John, for example, is entitled to call himself Sir Elton. The female equivalent is a Dame.
Accompanying the title is the given name, and optionally the surname. So, Elton John may be called Sir Elton or Sir Elton John, but never Sir John. Similarly, actress Judi Dench D.B.E may be addressed as Dame Judi or Dame Judi Dench, but never Dame Dench.