Republican era Legion
At optimum strength the legion of the Republic had 4,200 men. Each legio was made up of ten subdivisions of 420 men, known as a cohors ("cohort" in English). Each cohors consisted of one maniple of 60 triari, one of 120 princips, one of 120 hastati, and one of 120 velites. Each maniple was divided into 2 centuriae ("centuries" in English). Each comanded by a centurio, (centurion). These may have been divided further into units of 6 men called contuburnia

Imperial Roman Legion
Commanded by the legatus, the Legio at full strength was comprised of 6,000 men. It was divided into 10 cohortes (cohorts). Each cohors was made up of 480 men, with the exception of the First Cohort which had 960. A cohors was probably under the command of the most senior centurion of its centuries. Cohortes were divided into six 6 centuriae (centuries). Each centuria held 80 men and was led by the centurio (centurion). The centuries of the First Cohort held 160 men. The centuriae were divided into 10 contubernium. These were groups of 8 men which shared a tent. In addition there was a 120 man cavalry unit attached to the legion and possibly as many as 60 artillery pieces.

Fig. SL003 First century A.D. optio. Just bellow the rank of centurio the optio was noted on the battlefield by his double-plumed helmet and hastile (staff). Fig. SP001 1st century B.C. Melite.

Legatus Legionis The overall legionary commander. This post was filled by a man from the senatorial class picked by the emperor. He usually had previous military experience in command as a tribune. Typically, the man filling this post used it only a tem[porary step in his political career. So, unfortunately, it was very possible for a legion to be commanded by a legate who had little interest or ability in military matters. Tribuna Laticlavius Each legion had 6 military tribunes who were the sons of senators or other equestrians (knights). Normally the tribunes divided themselves into 3 pairs and took turns commanding in 2 month intervals. This post was used as a way to train for later being a legate. Praefectus CastorumThis man was responsible for maintenance of the legionary fortress, supplies, munitions, equipment, etc. This post was usually filled by a man in his 50s or 60s who had been promoted from the rank of Primus Pilus. When the legate and senior tribune were away, this man took command. Primus Pilus or Primipilus This was the centurion of the first cohort. He was the senior centurion of the legion. Centuriones (chohort I) These were the other 4 centiurions of the 1st cohort. Along with the Pimus Pilus they were know as the Prima Ordines Centuriones (chohorts II-X)There was one centurion for each century. Most of these men had previously served in the legion as soldiers or princepales, though some were appointed the position because of their status in the equestrian class Optiones 2nd or sometimes 3rd in command of a century because some Centuries had a 2nd Centurian then an Optio. The Optio carried a staff called a hastile - with an ornamental knob at top. In battle he was stationed at the rear of the advancing unit. Tesserarius 3rd in command of a Century. The tesserarius was traditionally responsible for commanding the sentries. The name comes from the tessera, the tablet that the night's password was distributed throught the camp (castra). Some centuries had more than one. They were responsible for the regular duties of the milites. Princepales These were the non commisioned officers appointed to aid the officers. Immunes Any soldiers who were exempt from the less desireable duties (such as latrine duty and digging ditches I presume) were called immunes. They had this special status because they held extra responsabilities such as being among the medical personnel, a musician, an instructor, etc. Vexillarius: in charge of carrying the vexillum, or battle flag. He was the highest ranked member of the immunes See article on the Vexilla. Imaginifer: carried a sculptural image of the emperor on a pole. Milites The "enlisted men" of the legion: common soldiers.