Ridicule of the military was popular, but everyone really knew that without the military, Rome would just be another dot on the map and easy prey for marauders and "barbarians". During the early Republic, the military, including the officer corps, was made up of citizen volunteers who were organized on an as-needed basis by top level commanders appointed by the Senate. But by about 100 BC, a more professional operation emerged. Individual military leaders, especially Consuls carrying out foreign wars (theater commanders), accumulated vast amounts of loot, and were able to keep large armies -- multiple legions -- under arms on a permanent basis. Their loyalties were to the generals who paid them rather than to the state.
This ultimately led to the civil wars that marked the last 50 years or so of the Republic. Warlord fought warlord. Julius Caesar and Pompey ultimately fought it out on the Italian Peninsula and in Greece using rival Roman armies. After Caesar won, he proved so inept as a political leader that he was killed by rival politicians (including Brutus, whom Caesar was obviously grooming as his successor), and the civil wars continued for another 14 years until Octavian (later called Augustus) finally defeated Mark Antony.
Among Octavian's first acts, with the joyous approval of the Senate and people of Rome, who were really tired of civil war, was to end the warlord system and establish a professional army paid by the state and sworn to loyalty to the Emperor rather than to a General. Octavian's plan held for a long time, and it only broke down when other things had already severely damaged the state.
So what was the Roman Army. The core of the force were the 25 to 34 Roman legions (legio simply meant a "levy") of five to six thousand Roman citizens each. Not all these citizen-soldiers were from Rome itself or even from Italy. As citizenship spread to the provinces, replacements in existing legions were often drawn from there. When new legions were formed, however, as they occasionally were, they almost always got their commissioning levies from Italy. Enlistment as a member of a legion was essentially for life -- twenty to thirty years at different times. If you survived your battles, you could eventually retire with a pension and some land, but you were already likely to be pretty much at the upper end of your life expectancy. Officers, especially the fast risers or those who started near the top because of family or political connections, could get an early semi-retirement, but even the top political generals might be called back to handle a crisis. Julius Caesar, for example was sent back to Gaul, which was still his command theater, to handle a local uprising at a critical moment in his rivalry with Pompey. It's why he was out of town and had to cross the Rubicon and fight his way back down to Rome.
Legions always had a number, a name, and a unit symbol or logo, but there was no apparent pattern in numeration or naming. Some legions were remnants of units split by the civil wars, so there evem could be more than one legion with the same name and number. They carried their symbol or logo along with the Imperial Eagles, and the shafts of unit markers were decorated with ribbons for previous battles and sometimes with body parts of previous enemies.
Legions were like modern Regiments. Almost all the troops in a legion were heavy infantry, but there were also cavalry, archers and/or elite light infantry, and artillerymen. Legions also included specialists: combat engineers, who organized all of the legionnaires to build and maintain their own fortifications, roads and bridges, surveyors, weapons makers, and administrators. The professional administrators often were the cadres for the provincial bureaucracy.
There were ten "cohorts" per legion and three "maniples" to a cohort. A "maniple" had two "centuries", each of which was commanded by a Centurion. As the name implies, a "century" of nominal strength had one hundred men, but few were ever of nominal strength, either because the legion had less than 6000 troops or because some centuries were strengthened and others weakened. Such decisions were local, and a good Centurion could have as many as 200 men while his weaker colleagues might only be trusted with 30. Almost always, "centuries" were divided into ten man squads called "contubernia". Specialized troops had their own sub-units that were sometimes attached at one level and sometimes at another. Legionary commanders were given a greater or lesser degree of organizational autonomy depending on the preferences of theater commanders. The officer and NCO rank structures were fairly regular throughout the Empire with few variations over the centuries. The system worked efficiently, so there was no pressure for change.
Auxiliary units, often specialized and using their own tactics, were sometimes supplied by local allies, but sometimes also were transported across the Empire to where their special skills were needed. Such units mostly had their own officers and were under the command of the theater commander and seldom attached to specific legions.
The Praetorian Guards were heavy infantry units that were, naturally, at the praetoria or headquarters. Augustus set up nine praetorian cohorts of about 500 men each and a tenth was soon added. By 100 AD, the strength of these praetorian cohorts was doubled, so they then had the strength of two legions. Praetorian cohorts were all usually barracked in their own large base at the northeast corner of Rome, which is today still called the "praetorian camp".
Rome also had three more "urban cohorts" who functioned as backup for the local police/fire units called "vigili" and were especially handy if the crowd got out of hand at the Circus Maximus or the Colosseum. The urban cohorts had a separate command structure from the praetorians, but both chains of command led to the imperial palace. The emperors also had about 1000 non-citizen mounted guardsmen, usually called batavi after the tribal auxiliary group from which they were originally drawn. They were trained and officered by top performers from the legionary cavalry units.
25 to 34 legions times 5000 to 6000 men each would mean about 125,000 to 200,000 legionnaires in the army at a time. The number of auxiliaries varied but was often higher than the number of legionnaires. Turnover was probably at least 100 percent every 20 years. Over say a three hundred year period from Augustus to Constantine (fifteen full turnovers), you would have more than 1.9 to 3 million regulars and maybe as many auxiliaries pass through the system. The regulars, most of whom were always heavy infantry, were issued standard swords, shields, helmets, spears, etc., most of which were eventually beaten into plowshares.
The Roman Army was much too complex to cover in a short article -- there are whole encyclopedias devoted to this one subject. Here are some Internet sites that will give you much more information:
Sander van Dorst's Roman Army Page: http://members.tripod.com/~S_van_Dorst/legio.html
(Also, scroll down for Navy info.)
Roman military terminology: http://members.tripod.com/~S_van_Dorst/glossary.html
Smith Dictionary articles on Warfare: http://www.ukans.edu/history/index/europe/ancient_rome/E/Roman/Texts/secondary/SMIGRA*/Warfare/home*.html
Roman military history links: http://www.legionxxiv.org/historylinks/. Westerners -- Germans and the Brits more than most -- seem to like to dress up like Romans and march around. This is a link to one of those "historical re-enactment" units, and it provides links to many more.
Roman military bibliography and links: http://www.csun.edu/~hcfll004/armybibl.html
Rome military resources: http://www.dalton.org/groups/rome/RMil.html
Military and Naval History from RomanSites: http://www.ukans.edu/history/index/europe/ancient_rome/E/Roman/RomanSites*/Topics/Warfare.html
The Army from Halsall's Ancient History Sourcebook:
Don't forget the Navy!
Navigare Necesse Est: http://www.romaeterna.org/english.htm
Navis database: http://www1.rgzm.de/Navis/Home/NoFrames.htm
Naumachia -- the naval games
Smith's dictionary -- Naumachia: http://www.ukans.edu/history/index/europe/ancient_rome/E/Roman/Texts/secondary/SMIGRA*/Naumachia.html
Drawing of Augustus's Naumachia on the Trastevere side
of the Tiber: http://academic.bowdoin.edu/classics/research/moyer/html/intro_naum.shtml