A Roman Legion during the Empire era consisted of roughly 5,000 men at full strength, though they were only rarely at this level. Legionaries had very effective weapons and armor, making them strong. [To find names of parts of armor and weapons, Google "Roman Armor/Weapons".]

A Somewhat More Detailed Answer:

During the Kingdom of Rome, the earliest known phase of Roman history, the legion did not exist, although its foundations were present. Soldiers were called up and grouped into units of 100, called centuries, as needed. These soldiers were armed in an almost identical fashion to the phalangites of Greece. The century became the foundation unit of the legion in later times. The Legions were first formed during the Republican period, and continued on through the Empire. Each legion had a unique symbol and name. The names of about 50 are known to modern historians, but there were hundreds of legions. Some of them had great reputations as elite forces, unbreakable and unbeatable. Others had bad reputations. One of the most famous was Julius Caesar's legendary Legio X Equestris (10th legion 'mounted', known as the Tenth Legion) famous for its ability in battle and its unswerving loyalty to Caesar.

A Roman Legion was the main fighting force of the Republic and later Empire of Rome. When the Roman legions reached their most recognisable form, after the Reforms of Marius, it was a formidably well organised, competent and effective fighting force. No European army from the fall of Western Rome to at least the 17th Century would rival the legions for organisation both on the battlefield and at higher levels. It was often this organisation that gave the legions their edge over almost every opponent, rather than bravery or weapons skill. The legions fought as a coordinated body, as a group and not as individuals. In situations that prevented the legions from operating in the usual smooth, coordinated fashions it could be and often was defeated.

A legion was one unit of the overall Roman army. Rome wielded many of them and numbers fluctuated, although growing as the Republic/Empire expanded. Each legion was commanded by a Legate, who had a system of officers under him to govern its component parts. During the Republic, the Legate was usually a Senator, chosen by the Senate, and sometimes a Consul. During the Empire, the Emperors would simply appoint them. Although the heart of every legion was the citizen infantry, the famous legionaries, a legion was actually a complete army including cavalry, artillery and a supply train. Each legion was capable of operating independently, although they could be used in groups. Auxiliary units of non-citizens supported the legions, consisting of light and ranged infantry and cavalry.

Discipline was severe in the legions, and any man who broke the rules could expect anything from extra camp duties to flogging or execution. For example, any unit that fled from the enemy could expect to be decimated - that is, 1 man in every 10 would be killed, often by his comrades. In some legions, the legionaries probably feared their commanders more than the enemy! On the other hand, the rewards were good. At the beginning, it was a mark of status to serve. Later, the pay was reasonable and completion of the term of service (25 years) would bring a gift of land somewhere in the empire and, eventually, Roman citizenship - in Ancient Rome, this last was sometimes the biggest incentive of all. Soldiers were also entitled to a percentage of profits from the sale of slaves and loot, and sometimes this could amount to small fortunes - particularly on campaign in the wealthy Middle East. From around 100 BC the legions followed an eagle standard and another one, unique to each legion, into battle, and would die to keep them safe. The standards were the heart of the legion, and would be decorated with medals of service and commendations the legion had won. It's loss was a humiliation and disaster. It was viewed as unlucky and a sign of divine disfavour, and some legions that lost their standards would even be disbanded, although this was often also because losses had been so heavy the legion was no longer able to engage in combat.

In the later phases of the Empire, the legions became a powerful political force. On multiple occasions, Emperors rose to power (or fell from it) as they gained or lost the support of the legions. Gradually, the quality of the legions and their commanders was degraded for many reasons, and in the end were no longer capable of protecting even the Eternal City, Rome itself. Even after the fall of the Western Roman Empire, the legions continued in service of the Eastern Roman or Byzantine Emperor. In the 7th Century A.D. Byzantine Emperor Heraclius reformed the army and the legions disappeared at last, leaving behind a centuries-old record of superb ability and influence.