Cavalry
Role: Historically, cavalry was divided into light cavalry and heavy cavalry. The differences were their roles in combat, the size of their mounts, and how much armor was worn by the mount and rider.
Heavy cavalry, such as Byzantine cataphracts and knights of the Early Middle Ages in Europe, were used as shock troops, charging the main body of the enemy at the height of a battle; in many cases their actions decided the outcome of the battle, hence the later term battle cavalry. Light cavalry, such as horse archers, hussars, and Cossack cavalry, were assigned all the numerous roles that were ill-suited to more narrowly-focused heavy forces. This includes scouting, deterring enemy scouts, foraging, raiding, skirmishing, pursuit of retreating enemy forces, screening of retreating friendly forces, linking separated friendly forces, and countering enemy light forces in all these same roles.
Light and heavy cavalry roles continued through early modern warfare, but armor was reduced, with light cavalry mostly unarmored. Yet many cavalry units still retained cuirasses and helmets for their protective value against sword and bayonet strikes, and the morale boost these provide to the wearers, despite the actual armour giving little protection from firearms. By this time the main difference between light and heavy cavalry was in their training and weight; the former was regarded as best suited for harassment and reconnaissance, while the latter was considered best for close-order charges. By the start of the 20th century, as total battlefield firepower increased, cavalry increasingly tended to become dragoons in practice, riding mounted between battles, but dismounting to fight as infantry, even though retaining unit names that reflected their older cavalry roles. Military conservatism was however strong in most continental cavalry during peacetime and in these dismounted action continued to be regarded as a secondary function until the outbreak of World War I in 1914.
With the development of armored warfare, the heavy cavalry role of decisive shock troops had been taken over by armored units employing medium and heavy tanks, and later main battle tanks. Despite horse-borne cavalry becoming obsolete, the term cavalry is still used, referring in modern times to units continuing to fulfill the traditional light cavalry roles, employing fast armored cars, light tanks, and infantry fighting vehicles instead of horses, while air cavalry employs helicopters.

Early history: 

Origins: Before the Iron Age, the role of cavalry on the battlefield was largely performed by light chariots. The chariot originated with the Sintashta-Petrovka culture in Central Asia and spread by nomadic or semi-nomadic Indo-Iranians. The chariot was quickly adopted by settled peoples both as a military technology and an object of ceremonial status, especially by the pharaohs of the New Kingdom of Egypt from 1550 BC as well as the Assyrian army and Babylonian royalty.
The power of mobility given by mounted units was recognized early on, but was offset by the difficulty of raising large forces and by the inability of horses (then mostly small) to carry heavy armor. Nonetheless, there are indications that, from the 15th century BC onwards, horseback riding was practiced amongst the military elites of the great states of the ancient Near East, most notably those in Egypt, Assyria, the Hittite Empire, and Mycenaean Greece.
Cavalry techniques, and the rise of true cavalry, were an innovation of equestrian nomads of the Eurasian Steppe and pastoralist tribes such as the Iranic Parthians and Sarmatians. Together with a core of armoured lancers, these were predominantly horse archers using the Parthian shot tactic. 

The photograph straight above shows Assyrian cavalry from reliefs of 865–860 BC. At this time, the men had no spurs, saddles, saddle cloths, or stirrups. Fighting from the back of a horse was much more difficult than mere riding. The cavalry acted in pairs; the reins of the mounted archer were controlled by his neighbour's hand. Even at this early time, cavalry used swords, shields, spears, and bows. The sculpture implies two types of cavalry, but this might be a simplification by the artist. Later images of Assyrian cavalry show saddle cloths as primitive saddles, allowing each archer to control his own horse.
As early as 490 BC a breed of large horses was bred in the Nisaean plain in Media to carry men with increasing amounts of armour (Herodotus 7,40 & 9,20), but large horses were still very exceptional at this time. By the fourth century BC the Chinese during the Warring States period (403–221 BC) began to use cavalry against rival states, and by 331 BC when Alexander the Great defeated the Persians the use of chariots in battle was obsolete in most nations; despite a few ineffective attempts to revive scythed chariots. The last recorded use of chariots as a shock force in continental Europe was during the Battle of Telamon in 225 BC. However, chariots remained in use for ceremonial purposes such as carrying the victorious general in a Roman triumph, or for racing.
Outside of mainland Europe, the southern Britons met Julius Caesar with chariots in 55 and 54 BC, but by the time of the Roman conquest of Britain a century later chariots were obsolete, even in Britannia. The last mention of chariot use in Britain was by the Caledonians at the Mons Graupius, in 84 AD.

Ancient Greece: city-states, Thebes, Thessaly and Macedonia: During the classical Greek period cavalry were usually limited to those citizens who could afford expensive war-horses. Three types of cavalry became common: light cavalry, whose riders, armed with javelins, could harass and skirmish; heavy cavalry, whose troopers, using lances, had the ability to close in on their opponents; and finally those whose equipment allowed them to fight either on horseback or foot. The role of horsemen did however remain secondary to that of the hoplites or heavy infantry who comprised the main strength of the citizen levies of the various city states.
Cavalry played a relatively minor role in ancient Greek city-states, with conflicts decided by massed armored infantry. However, Thebes produced Pelopidas, their first great cavalry commander, whose tactics and skills were absorbed by Philip II of Macedon when Philip was a guest-hostage in Thebes. Thessaly was widely known for producing competent cavalrymen, and later experiences in wars both with and against the Persians taught the Greeks the value of cavalry in skirmishing and pursuit. The Athenian author and soldier Xenophon in particular advocated the creation of a small but well-trained cavalry force; to that end, he wrote several manuals on horsemanship and cavalry operations.
The Macedonian kingdom in the north, on the other hand, developed a strong cavalry force that culminated in the hetairoi (Companion cavalry) of Philip II of Macedon and Alexander the Great. In addition to these heavy cavalry, the Macedonian army also employed lighter horsemen called prodromoi for scouting and screening, as well as the Macedonian pike phalanx and various kinds of light infantry. There were also the Ippiko (or "Horserider"), Greek "heavy" cavalry, armed with kontos (or cavalry lance), and sword. These wore leather armour or mail plus a helmet. They were medium rather than heavy cavalry, meaning that they were better suited to be scouts, skirmishers, and pursuers rather than front line fighters. The effectiveness of this combination of cavalry and infantry helped to break enemy lines and was most dramatically demonstrated in Alexander's conquests of Persia, Bactria, and northwestern India.

Roman Republic and early Empire: The cavalry in the early Roman Republic remained the preserve of the wealthy landed class known as the equites—men who could afford the expense of maintaining a horse in addition to arms and armor heavier than those of the common legions. Horses were provided by the Republic and could be withdrawn if neglected or misused, together with the status of being a cavalryman.
As the class grew to be more of a social elite instead of a functional property-based military grouping, the Romans began to employ Italian socii for filling the ranks of their cavalry. The weakness of Roman cavalry was demonstrated by Hannibal Barca during the Second Punic War where he used his superior mounted forces to win several battles. The most notable of these was the Battle of Cannae, where he inflicted a catastrophic defeat on the Romans. At about the same time the Romans began to recruit foreign auxiliary cavalry from among Gauls, Iberians, and Numidians, the last being highly valued as mounted skirmishers and scouts (see Numidian cavalry). Julius Caesar had a high opinion of his escort of Germanic mixed cavalry, giving rise to the Cohortes Equitatae. Early emperors maintained an ala of Batavian cavalry as their personal bodyguards until the unit was dismissed by Galba after the Batavian Rebellion.
For the most part, Roman cavalry during the early Republic functioned as an adjunct to the legionary infantry and formed only one-fifth of the standing force comprising a consular army. Except in times of major mobilisation about 1,800 horsemen were maintained, with three hundred attached to each legion.