Wikipedia
The Auxilia ( Latin, lit. "helps") constituted the standing non-citizen corps of the Imperial Roman army during the Principate era (30 BC–284 AD), alongside the citizen legions. By the 2nd century, the Auxilia contained the same number of infantry as the legions and in addition provided almost all of the Roman army's cavalry and more specialised troops (especially light cavalry and archers). The auxilia thus represented three-fifths of Rome's regular land forces at that time. Like their legionary counterparts, auxiliary recruits were mostly volunteers, not conscripts.
The Auxilia were mainly recruited from the peregrini, i.e. free provincial subjects of the Roman Empire who did not hold Roman citizenship and constituted the vast majority of the empire's population in the 1st and 2nd centuries (c. 90% in the early 1st century). The Auxilia also included some Roman citizens and probably barbarians (barbari, as the Romans called peoples located outside the Empire's borders). This was in contrast to the legions, which admitted Roman citizens only.
Roman auxiliary units developed from the varied contingents of non-Italian troops, especially cavalry, that the Roman Republic used in increasing numbers to support its legions after 200 BC. The Julio-Claudian period (30 BC–68 AD) saw the transformation of these motley temporary levies into a standing corps of regiments with standardised structure, equipment and conditions of service. By the end of this period, there were no significant differences between legionaries and most auxiliaries in terms of training, or thus combat capability.
Auxiliary regiments were often stationed in provinces other than the province in which they were originally raised, both for reasons of imperial security and to foster the process of Romanisation and integration of the provinces. The regimental names of many auxiliary units persisted into the 4th century, but by then the units in question were different in size, structure, and quality from their predecessors.