The Cohort was the basic unit of the Roman Army. A Cohort was usually composed of 6 to 10 Centuries, which contained 60 to 80 men. New recruits were placed in the lowest numbered century, and worked their way up through experience. Centuries were ran by Centurians, who  were comparable to today's 1st Sergeants. The best and most experienced Centurians commanded the 1st Century, and the Centurians of the lower Centuries were expected to follow him. Centuries were commanded by Tribunes, who usually came from the ranks of Roman aristocracy, except for the rare case where a Centurian was promoted to Tribune by displaying exceptional leadership skills.

The Roman army also had an exceptional training system, with the most experienced Centurians acting as drill sergeants. This system gave the Legions their world renouned discipline, which modern armies have copied to this day.
Combined with their superior communications and supply systems, these elements combined to give the Romans the tools to form the ancient world's greatest empire.