Structure of the Roman Army

The Roman army was broken down into different groups to have a clear chain of command during battle.

The smallest unit was the conturbenium, which was a group of eight soldiers. These men marched together and shared a tent or a room at a fort. Ten conturbenium made up a century (only eighty men on average) which was commanded by a centurion. Six centuries would be combined to make up a cohort. Then ten Cohorts would be combined to make up a legion or about 6,000 men.

The first cohort of the legion was usually twice the size of a normal cohort, and had the best soldiers in it. The legion, all of which was infantry, was the backbone of the Roman army. "Each legion contained four lines, or groups, of soldiers. The front line soldier was the velites, who was trained to throw spears at the enemy. Behind the velites was the hastatus and the preinceps. These soldiers did most of the fighting. They had light armor and used swords. The last line was the triarius, who wore heavy armor". In addition to the legionaries, auxiliary cohorts of cavalry or specialists such as archers, would also be part of the unit.

See http://historylink102.com/Rome/roman-army-structure.htm