Called miles ("soldier") or legionarius in Latin, the Romanlegionary was (usually) a Roman citizen under 45 years of age. The soldier enlisted in a legion for twenty-five years of service, a change from the early practice of enlisting only for the duration of a campaign. The last five years were on veteran lighter duties.
The Roman soldier was trained especially hard, discipline was the base of the army's success and the soldiers were relentlessly and constantly trained both with weapons but especially with drill - forced marches with full load and in tight formation were frequent. Discipline was important and infractions were heavily punished by the centurions.
All in all the Roman army consisted of 18 centuries of equites, 82 centuries of the first class (of which 2 centuries were engineers), 20 centuries each of the second, third and fourth classes and 32 centuries of the fifth class (of which 2 centuries were trumpeters).
The Romanlegions might have been defeated by Pyrrhus (and only survived due to a near endless resource of fresh troops) but the experience gathered by fighting such an able foe was to prove invaluable for the great contests that lay ahead.
Roman society was governed by class and so in effect there was three separate army careers possible, that of the common soldier in the ranks, that of the equestrians and that for those destined for command, the senatorial class.
In a Roman province with only one legion, the legatus was also the provincial governor and in provinces with multiple legions, each legion had a legatus and the provincial governor had overall command of them all.
Many legionaries became devotees in the cult of the minor goddess Disciplina, whose virtues of frugality, severity and loyalty were central to their code of conduct and way of life.
The influence of Roman military and civic culture, as embodied particularly in the heavy infantry legion, gave the Roman military consistent motivation and cohesion.