The Title of "Centurion" was used throughout Ancient Rome's Military. The Position changed with each "new" Emperor. However, the basic duties of a Centurion were unchanged. Originally, a Centurion was a professional who commanded The Roman Formation known as a "Century" (or 100 men). However Higher Ranking Centurions could also command up to 1,000. Centurions were most often "created" by The Roman Senate. They could be promoted for bravery on the battlefield. Centurion were paid much better than a typical Roman Legionaire (as much as 17 times more).
A Centurion most often fought alongside his soldiers, in the Front Formations. As such, they suffered a "high mortality rate". Centurion were identified by a colored Horsehair Crest attached to his Helmet. Additionally, The Centurion had better Armor, and was the only Roman Soldiers "allowed" to wear his Sword on his "left side". In the later and waning Period of The Empire, Centurions were more "Political" positions, often paid for by wealthy Romans - for their Sons. A Military "background" was often not seen as necessary for promotions to certain Civil positions within The Empire.
Much has been made in Modern Eras, regarding just "what" The equivalent "Rank" of a Centurion would be. They could range anywhere from Non-Commissioned Officer to the Rank of "Colonel". The term "Centurion" was not seen much in use after the 6th Century AD.