Traditionally, Roman armies are made up of Roman citizens, paid volunteers who joined for defined terms of service. Recruitment is handled by the generals, and quite frequently soldiers are paid with funds raised during combat.
The largest unit of the Roman army is the legion. At full strength (which is a rare condition, of course), a legion has six thousand men, divided into ten cohorts of six hundred each. In turn, cohorts were made up of three maniples of two hundred men, and each maniple of two centuries of - you guessed it - one hundred men. At the head of each century is the centurion, a non-commissioned officer promoted from the regular ranks. Most of the other officers were members of the patriciate, as it is customary for young patricians to serve as officers for a couple years before embarking on a political career. The generals, who usually take charge of a legion each, are men of senatorial rank. Each legion also has six military tribunes, who are appointed by the popular assemblies.
Along with the regular legions, Roman armies use auxiliary infantry, which are made up of locals. There is also a cavalry force; although originally this was made up of Romans of the equestrian rank, these days it is more likely to use locals. Caesar's cavalry is at present entirely Gallic, for example.
Legionaries wear a wool tunic under a leather coat, with bands of metal for added protection. They use a cloak as protection from the elements, and wear a metal helmet and carry a large, wooden, rectangular shield. They have two weapons. The spear was long and is used for throwing; it's designed to bend when it hits an enemy's shield, so that it can't be thrown back. The sword is double-edged and pointed.
Rome relies heavily on slave labour. Slaves tend to be a result of military conquests. For example, it is customary for a victorious general to sell his prisoners into slavery. Slavery is an inherited condition. Masters do, of course, have the ability to free their slaves, which is sometimes done in cases of exceptional service. Also, in times of military emergency, slaves can be convinced to enlist in the army in exchange for freedom when their term is complete.
The mass of urban poor. Plebians have little electoral power and even less chance of social or economic advancement. However, they do enjoy some of the advantages of Roman citizenship, such as the government-subsidized grain supply.
Originally, the class of people who could afford their own horses. Now, the equestrian order is essentially the new rich: they are non-aristocratic families who have come into wealth. Their voting rights are inferior to those of the patricians, but nonetheless they are a very important group in Roman politics. Equestrians are often enterprising businessmen of one sort or another. They are often called "new men." (homines novi) Cicero, for example, is of the equestrian order.
The Roman aristocracy. The patricians are the most powerful group in Roman society and politics. Generally, the only profession they involve themselves with is politics.
Rome is a Republic. Power is divided between the Senate, the popular assemblies, the magistrates, and the Tribunes of the People.
Heading the executive branch of Roman politics, magistrates are divided into the following groups, in order of descending importance: consuls, praetors, aediles, quaestors...**** These positions are elected yearly. The consuls, of which there are two per year, preside over the senate, can propose legislation, and are expected to oversee military operations. At the other end of the scale, there are 20 quaestors a year, and they occupy themselves with financial and administrative duties of various sorts.
The Senate is the most powerful body in Roman government. It is composed of ex-magistrates, and is generally expected to guide the Republic. Senators hold their position for life. There were originally 300 senators; however, the number was recently raised to 600. Traditionally the Senate is made up of patricians, but there are a few equestrian Senators as well. The Senate almost always takes a conservative, patrician position, but tries to make concessions and compromises when necessary.
The last few decades have seen an increase in the power of the popular assemblies. This is generally because the Senate tends to grant concessions in times of plebian revolt. Now, the popular assemblies can pass resolutions that have the force of law.
The Tribunes are meant to safeguard the rights of the plebians. There are 10 a year. Their strongest power is their right of veto over any law passed by the Senate.
Some consider the Roman government extremely lean. We do not have a large bureaucracy. We tend to let private citizens handle many of the traditional duties of government, including policing, tax collection, and the administration of public works.
We do not have "political parties." Networks of associations are formed with respect to specific issues. Favours are exchanged. A politician who wishes to succeed must win friends by helping them in the courts, helping them financially, or promising votes to them. There is a strong sense of voting allegiance, but it is tied to friends, patrons and clients rather than parties.