A paramilitary is a group of civilians trained and organized in a military fashion.
Paramilitary groups can serve many different functions. Some are created by governments as internal security forces. Some are formed as revolutionary groups, or engage in guerilla warfare, and may fight against opposing government paramilitaries. Some are commando units, created by a state and intended for non-traditional combat missions, operating outside the military. Other paramilitary groups adopt military organization and aspects of military culture and discipline, but are not intended to see any form of combat.
Paramilitary groups as security forces
These groups are neither a police agency nor a military organization, having elements of both but also lacking elements of both. Much of the world's military forces, particularly in developing countries, could be considered paramilitary; they are oriented towards controlling their own country's population rather than toward the role of a professional military.
This sort of paramilitary force typically exists to assure the internal control of a country and to suppress anarchy or civil war. They are typically armed with small arms and wear military uniforms. They are also often equipped with tear gas and other non-lethal weapons. Such paramilitaries may be controlled by the ruling political party or by the head of state personally rather than by the government.
In some situations, where the state or military apparatus is particularly weak or absent, they can act with a large degree of practical independence, having their own command structures and benefitting from private sponsors (instead of, or in addition to, any institutional ones), such as landowners, regional authorities, local interest groups, former victims of revolutionary paramilitary forces, warlords, drug lords or foreign interests. These sponsors may then be able to further extend their influence or control over the paramilitary forces, or even organize paramilitary groups of their own.
These forces ostensibly operate to enforce the law but may act with disregard of the law or at cross-purposes to the existing civilian or military authorities, which may or may not lead to confrontation if the resulting discrepancies are significant enough.
Depending on their degree of political and financial autonomy, the relationship between the paramilitary forces and official institutions can be one of tolerance or incidental alliances rather than of direct oversight and cooperation. These groups may then act according to their own subset of tactical, economical and even political objectives, which may or may not be in opposition to those of the central government or established military command as a whole. Paramilitary forces have been responsible for some violations of the laws of war and for several atrocities.
In military terms, paramilitary security forces are typically light infantry. Effectively led they can stand in defense, especially in cities or urban areas, but are less capable of offensive action or sustained combat operations due to a lack of heavy weapons, professional military training, and effective logistics support.
Examples of this kind of paramilitary force include the People's Armed Police in the People's Republic of China which was split off from the People's Liberation Army in 1983 precisely to remove paramilitary duties from the PLA, Colombian right wing paramilitary groups such as the AUC, and the East German Kampfgruppen der Arbeiterklasse. One may also consider that US police Special Weapons And Tactics units, or similar units in other countries, are paramilitary.
Certain countries, following the French model have a Gendarmerie – a national police force with military status, responsible for law enforcement in rural areas and military installations. In the case of countries with a rule of law, such forces, however, are not referred to as paramilitary except in polemical fashion. The largest part of the gendarmeries is made of "normal" officers who perform duties in a way similar to what a Sheriff would do in the United States.
Revolutionary and guerilla paramilitary groups
Some paramilitaries are formed to fight the current government of a country or region. This includes rebellions against recognized governments and attacks on occupying forces. While some insurrections are carried out by rebellious units of a country's military, many are staged by paramilitary groups. When a group is acting locally against a military occupation, its members may be referred to as partisans, guerrillas, or as resistance fighters by supporters. The military occupation forces will generally refer to them as terrorists.
Unlike state security paramilitaries, these groups are typically engaged in asymmetric warfare against an established and stronger force. In fact, they may be fighting both against the government and against other paramilitaries that support or are controlled by the government.
Examples include FARC and ELN in Colombia, and Hamas, Huzbollah, Islamic Jihad, Fatah and other armed insurgent groups in the Middle East, the Lord's Resistance Army of Uganda, the IRA in Ireland and ETA in Spain.
In some instances, paramilitary groups have worked to destabilize and overthrow democratic governments, generally in support of a fascist regime or, at the contrary, of a Communist revolution. As a consequence, many jurisdictions have laws prohibiting private paramilitary groups.
Examples include the Sturmabteilung (helped install Nazi Germany), the Blackshirts (helped install Fascism in Italy).
In some cases, paramilitary groups are formed to perform commando functions. Unlike internal security forces and revolutionaries, commando paramilitary groups are generally small and highly trained. The paramilitary operations of the CIA and Mossad (as distinct from their intelligence-gathering function) are one example. Police SWAT teams are another.
Paramilitary groups as mercenaries
Paramilitary groups may also be formed to serve as mercenaries or private armies. Among the best-known of such groups is Executive Outcomes, a mercenary corporation that operated in Africa in the 1990s. Such groups are often made up of former military personnel, especially former special forces soldiers.
Private firms have recently gained a major role in the operation of Western militaries, especially that of the United States. Such private military companies played key roles in the 2003 invasion of Iraq and the subsequent occupation, doing jobs like logistics and security.
See the SourceWatch article: Private Military Corporations
Non-combat paramilitary organizations
Because "paramilitary" refers to the organization, not the purpose, of a group, some groups could be called paramilitary whose purpose is not to fight. These groups adopt some aspects of military culture and military discipline, such as military courtesy or a strict hierarchy. In wartime, groups like these may be pressed into combat because they have the necessary discipline and organization.
Examples include various cadet corps, the Hitler Youth, the US Civil Air Patrol and the United States Coast Guard Auxiliary.