Historically, cavalry (from the French word cavalerie, itself derived from cheval meaning "horse") are groups of soldiers or warriors who fight mounted on horseback. Until the 20th century, cavalry were the most mobile of the combat arms, operating as light cavalry in the roles of reconnaissance, screening, and skirmishing, or as heavy cavalry for decisive economy of force and shock attacks. An individual soldier in the cavalry is known by a number of designations depending on era and tactics, such as a cavalryman, horseman, trooper, cataphract, knight, drabant, hussar, uhlan, mamluk, cuirassier, lancer, dragoon, samurai or horse archer. The designation of cavalry was not usually given to any military forces that used other animals or platforms for mounts, such as chariots, camels or elephants. Infantry who moved on horseback, but dismounted to fight on foot, were known in the early 17th to the early 18th century as dragoons, a class of mounted infantry which in most armies later evolved into standard cavalry while retaining their historic designation.

Cavalry had the advantage of improved mobility, and a soldier fighting from horseback also had the advantages of greater height, speed, and inertial mass over an opponent on foot. Another element of horse mounted warfare is the psychological impact a mounted soldier can inflict on an opponent.

The speed, mobility, and shock value of cavalry was greatly valued and exploited in warfare during the Ancient and Medieval eras. Some hosts were mostly cavalry, particularly in nomadic societies of Asia, notably the Huns of Attila and the later Mongol armies. In Europe, cavalry became increasingly armoured (heavy), and eventually evolving into the mounted knights of the medieval period. During the 17th century, cavalry in Europe discarded most of its armor, which was ineffective against the muskets and cannons that were coming into common use, and by the mid-18th century armor had mainly fallen into obsolescence, although some regiments retained a small thickened cuirass that offered protection against lances, sabres, and bayonets; including some protection against a shot from distance.

In the interwar period many cavalry units were converted into motorized infantry and mechanized infantry units, or reformed as tank troops. The cavalry tank or cruiser tank was one designed with a speed and purpose beyond that of infantry tanks and would subsequently develop into the main battle tank. Nonetheless, some cavalry still served during World War II (notably in the Red Army, the Mongolian People's Army, the Royal Italian Army, the Royal Hungarian Army, the Romanian Army, the Polish Land Forces, and German light reconnaissance units within the Waffen SS).

Most cavalry units that are horse-mounted in modern armies serve in purely ceremonial roles, or as mounted infantry in difficult terrain such as mountains or heavily forested areas. Modern usage of the term generally refers to units performing the role of reconnaissance, surveillance, and target acquisition (analogous to historical light cavalry) or main battle tank units (analogous to historical heavy cavalry).

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.