This morning before going to work I was watching some show on The History Channel about the Roman war machine. It mentions the Centurians as being the most famous of the Roman officers. Being the leader of 80 men I suppose their function was similar to a modern Captain, in the US T&O anyway, but the discription of these men as being career long term soldiers, the most experienced in the army, make them sound more like sergeants. I don't know anything about the Roman army. Were these men more like modern officers or senior NCOs?

If my memory of my Roman History courses is correct (don't bet the house on it #:-) ) there was no real level of command between the standard roman foot soldier and the centurion. The centurion held a position of authority like a Junior officer but also did much of the work that NCOs would do today. Older, experienced campaigners also did a lot of the same sorts of things like NCOs do today, just without any official rank to signify their seniority.
I'm sure someone else will post a more authoritative answer to this question, and if I get a chance I'll check some references and add anything else I find.
Hope this helps a little!

"In the early Republic, the legion of the Roman army was a self-contained formation equivalent to a complete army in itself. The smallest unit was the contubernium, a tent party of eight men sharing a packhorse. Ten such groups formed a company, or centuria, commanded by a centurian. This was originally 100 men, but normally numbered 80. Six such companies, grouped in three pairs, formed a cohort. Ten cohorts made up a legion."

From and website on the organization of the Roman Legion. So the Centurian was more of a commissioned officer than a NCO.

The first issue of "Armchair General" magazine had a small write-up about Roman Centurions that did a good job of concisely discussing the subject. One quote sums up well what a Centurion was:

"They were renowned for their bravery in battle and were expected to lead their troops by personal example. A good centurion led from the right-front of the formation and expected his soldiers to follow him and 'do as I do.' Akin to a modern-day first sergeant or command sergeant major, the Centurion reinforced discipline throughout the ranks of his unit and was respsonsible for everything his unit did or failed to do."

Centurions could be promoted to command an auxilliary unit or, even better, to Chief Centurion and put in command of the first century of the first cohort and thus in command of the cohort itself. This would be much more akin to the role of a modern officer than that of the Centurion in command of an 80 to 100 man formation.
As I said in my first post here, the Centurion can't be pigeon-holed as being either an officer or NCO. They essentially served both roles when starting out.
There is no author listed for the magazine article, but they did a pretty good job with the subject.

Now this is a subject bearing research. Was there a cohort commander? I take it cohorts is what made up a Legion or was there something in between? Was there a Legion commander? Was there another level above Legion?

According to Adrian Goldworthy's "The Punic Wars," a Roman Legion was a complex organization. The basic fighting unit in the Roman army was a maniple made up of two Centuries for a total of about 120 to 160 men. Each Century was commanded by a Centurion with the Centurion in charge of the right-hand century being in command of the maniple. The Optio was second in command to the Centurion and stood to the back of the formation and made sure the lines were properly dressed, much like a top NCO would do, but he was considered an officer, as was the signifer who carried the standard. A cohort was composed of three Maniples with the Centurion in command of the right-hand maniple serving as commander of the cohort. A typical Legion was composed of 10 Cohorts plus cavalry and velites, which were light infantry units composed of citizens too poor or young to join the legions and performed as skirmishers. All told, a typical legion had 3,000-4,000 Legionaires, 1,200 Velites, and around 300 cavalry. The legions were commanded by 6(!) elected military tribunes, who were normally aspiring politicians, although some had previous military experince. Pairs would take turns in command of the Legion, which would usually have a similar sized supporting unit of allied troops.
Any man could be promoted to any leadership rank except tribune for bravery in battle or leadership ability.
This system underwent various changes as time progressed, but this is how the Roman Legions were organized in the 3rd Century BC during the Punic Wars.