Legatus (Senatus): Commanded a Legion.
Tribunus Laticlavus (Senatus): Senatorial Tribune, a Magistrate of Rome.
Praefectus Castra: Quatermaster, Promoted from Primipilus. His civilian counterpart was the Praefectus Fabrum.
Tribuni Angusticlavi (Eqiutes): Equestrian Tribunes, commanded a Calvary unit.
Primipilus Centurion: The senior Centurion. Commanded the first Cohort and it's senior Century.
Tribuni Cohortis (Unelected): Commanded a Aux unit or had other various duties.
Tribuni Rufulus (Appointed): Appointed by the commander, had various duties.
Pilus Prior Centurions: Commanded a Cohort and it's first Century. Ranked in seniority the senior most is the Primipilus.
Centurio: Commanded a Century.
Optio: Later added as a rear rank officer to help the Centurio. Training to become a Centurio.
Contubernalis: A Cadet attached to the Legate's staff. Usually a noble born young man starting his political career.
Aquilifer: The Legions standard bearer (the Eagles). The best man in the Legion.
Signifer: The Century's standard bearer. The best man in the Century.
Imago: Carried an image of the Emperor.
Vexillifer: Carries the Detachments Vexillum.
Cornicen: The Century's signal/bugle player.
Tesserarius: A senior soldier (nco) in charge of guard duties.
Decanus: A senior soldier (nco) in charge of a 8 soldiers and 2 Volones (a tent group).
Legionary/Milites Gregari: A commom soldier.
Probatus: A trained recruit.
Tiro: A recruit in training.
Cohorts Rank Quick Chart
For more on Roman Legion organization click on this Wikkipedia link.