The terms are often confused, and often needlessly distinguished. The term knighthood comes from the English word knight (from Old English cniht, boy, servant, cf. German Knecht) while chivalry comes from the French chevalerie, from chevalier or knight (Low Latin caballus for horse). In modern English, chivalry means the ideals, virtues, or characteristics of knights. The phrases "orders of chivalry" and "orders of knighthood" are essentially synonymous.
The German translation for "knight" is Ritter (literally, rider). The Latin term in the Middle Ages was miles, since a knight was by definition a professional soldier. In modern times, the Classical Latin term eques was preferred.
Succintly, a knight was a professional soldier. The old "citizens' armies" of Antiquity had been replaced by professional armies. This trend was reinforced by the appearance in the 8th century of the stirrup, which made mounted men much more powerful and turned cavalry into the most important element of medieval armies. But being a mounted soldier was expensive, since it required enough income to buy and sustain a horse and the equipment (armor, weapons) to go with it. Thus, those who were too poor to provide this service became mere peasants, attached to the land.
In feudal society as it emerged in the 10th century, everyone held land from someone else in exchange for goods or services of some kind. Men who were not free provided a portion of their crops and labor services. Men who were free provided military service, either personally or (if they were rich enough) using others' services. Thus, a man who held his estate in knight's fee owed service as a knight to his lord. A more sizeable vassal, when called by his liege, would summon his knights and form a contingent in his liege's army.
Knighthood was originally a professional association. It included those men who could afford to make and maintain the heavy capital investment required by mounted warfare (horse and armor). It emerges in the 11th century, and its members are nobles (members of the great land-owning families) as well as small land-holders, free men, craftsmen, etc (in Spain, caballeros villanos were common until the 14th c.). It must be understood that, even in the feudal era, the boundaries of knighthood were quite fluid. Anyone who, by luck or effort, managed to obtain the training and equipment to be a knight, could eventually enter that class. In Flanders, there is a famous case of a family of servile (i.e., unfree) origin who entered into knighthood and became castellans of ??? in the 12th c.
In the course of the 12th century, a social and ethical dimension is added to this professional aspect. The strong influence of Cluny monks, who try to give an ethos to savage warfare, leads to the definition of the true miles Christi, a soldier who follows a certain code of behavior, which we now call chivalric. Starting in the second half of the 12th century, literature (gests and Arthurian romances) also provides a model for the knightly community, as well as a means of glorifying it.
Thus, knights were not necessarily nobles, nor were nobles necessarily knights. The noble class and the knightly class slowly came to merge from the late 12th century onward. Nobles become knights with increasing frequency. The French prince (future king Louis VI) was knighted without the knowledge of his father who remains distrustful of a rather heterogeneous professional class, but thereafter every French king is knighted (Favier 1993). Conversely, heredity enters the knightly class in the 13th century. The son of a knight is automatically a squire, thus making him eligible for knighthood on the basis of his ancestry; at the same time, knighthood is more and more restricted to descendants of knights by various legal restrictions imposed over the course of the 13th century. In the late 13th century, a decision of the Parliament in Paris forbade the count of Artois from making unfree men into knights without the king's consent; interesting to note, the two men who had been so knighted were allowed to remain knights subject to the payment of a fine. This marked both the closure of the knightly class as well as the beginnings of a new form of access, by purchase.
In England, the evolution was different: those who held land in knight's fee but did not wish to take up the profession could pay a tax. Knighthood did not become a hereditary class in England, and instead the knightly class (those eligible to be knights) became the nucleus of the gentry.
See also my page on women knights.
As a military institution, knighthood was on the wane from the late 13th century on. The end of feudal society meant that sovereigns gained a monopoly on war-making, and the old form of military service owed to one's immediate lord became obsolete. Kings still summoned their knights for wars, but increasingly they turned to other sources of manpower, namely mercenaries whose use became common in the 14th century. The war preparations of Henry V of England, which are well-documented, show how the king formed an army: he signed dozens of contracts (or indentures) with individuals who pledged to provide a specified number of men-at-arms and archers (usually 3 archers for each man-at-arm) at muster time.
The development of gunpowder and increasingly more powerful archery meant that the use of massive cavalry charges to break enemy lines and carry swift victory could not be relied upon, and the dominance of cavalry came to an end. If any battle summed up this change, it was the battle of Agincourt in 1415. The charging French knights, compressed by the terrain and the English arrows into a fragmented and ever constricted line of attack, reached the English line without any room to maneuver, and it only took a few fallen horses to prevent all other knights from moving in any direction. Thus, in half-an-hour the battle was decided, and thousands of French knights lay prisoners. The fear of a second attack prompted the English to kill them on the spot, and the French nobility was horribly decimated in a single day. The French learned their lesson; Charles VII, who finally expelled the English, formed the first standing, professional army in Europe.
The chivalric ideals continued to live on, perhaps precisely because the reality of knighthood had disappeared, and a free rein was given to romanticizing. The French king François Ier insisted on being knighted on the battlefield of his first victory at Marignano in 1515. Tournaments, pas d'armes were favorite entertainment at the French court of the 16th century. More and more elaborate suits of armor were forged for pure display, in increasingly baroque imitations of earlier models. Ariosto's poetic retelling of the crusades popularized the figures of Orlando and Ruggiero and extended the knightly myth for another 200 years. In the 19th century, when no one read Ariosto anymore, Sir Walter Scott and Romanticism took up the cause.
The origins of orders of knighthood are in the Crusades. In the Latin Orient, a new institution emerged, in which knights (professional soldiers) associated themselves under a strict, quasi-monastic rule of life, for the purpose of protecting pilgrims and defending Christian conquests in the Holy Land. In the 14th century, just as the original military-monastic orders were searching for a new mission after the loss of the Holy Land, kings began creating orders of their own, modelled in part on these original orders, but with a different purpose, to bind their nobility to themselves. Still later, in the late 16th century, these monarchical orders were imitated in form by the new orders of merit which became common throughout Europe.
Because each institution tried to use the prestige of the previous one by imitating it, the term "order of knighthood" has been passed on and is now used for modern awards and decorations which are neither orders nor composed of knights. In modern society, only a very few orders survive from the times of the Crusades, and most "orders of knighthood" awarded by sovereigns or governments (such as the English Garter or the Spanish Golden Fleece) are, in spite of their historical connection, awards of merit.
I discuss orders of knighthood at greater length.
The relations between heraldry, nobility and knighthood are often completely misunderstood. Briefly stated, heraldry appeared in the landed aristocracy and quickly spread to the knightly class in the 12th century, at a time when knighthood and nobility remain very distinct classes. Over the course of the 13th century, knighthood and nobility came to merge, just as heraldry spread far beyond either class to be used by all classes of society. Thus, heraldry is not particularly linked to nobility, although the most easily documented uses of heraldry are among nobles, simply because nobles were the elite.
The initial development of heraldry certainly owes a lot to the practices of the knightly class, in particular the growing fashion of tournaments, which became more and more popular from the 13th century, just as knighthood as a military institution was on the wane. Tournaments were the occasion to display coats of arms, and heralds, who were originally a specialized group of minstrels, became responsible for identifying and cataloguing the arms of participants. Their knowledge of coats of arms also helped them identify fighters in battle and dead on the battlefield, and for this reason heralds became associated with battles, truces, declarations of war, in an official capacity.