The Roman Legion generally refers to the whole of the military of the Roman Republic and Roman Empire, which was the most powerful in the world at the time. Technically, a legio (where we get the word "legion") was comprised of 5,000 to 6,000 men, while Rome's army consisted of several legiones, perhaps as many as 20 at a time, each led by a Legatus Legionis appointed by the Emperor himself. Thus, a Legatus Legionis was basically equivalent to a modern General.

Within each legion were about 50 or 60 centuriae, each led by a Centurion; it was called a centuria because it had about 100 men (thus our word "century" meaning 100 years). This makes a Centurion about the same rank as Lieutenant or Captain today. There weren't as many layers in between (today's Commander, Colonel, etc.) because Rome didn't have as large a population to deal with as we do now.

The Roman Legion was not only very large, they were also the best-equipped and best-trained soldiers in the world. As a result they were able to conquer most of Europe. Their power ultimately backfired, however, as Julius Caesar used his loyal and well-trained soldiers to effect a military coup and overthrow the republic. Shortly thereafter the Roman Empire was born.