What was the Roman Legion?
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.