A military or military force (n., from Latin militarius, miles "soldier") has seen many different incarnations throughout time.
Early armies may have been just men with sharpened sticks and rocks; through time they have included advancements such as men mounted on horses, men wielding swords and other metallic weapons, the bow and arrow, siege weapons, to the advance of the musket which form the roots of the armed forces of most nations we know today. In modern times people use vehicles and guns.
While military can refer to any armed force, it generally refers to a permanent, professional force of soldiers or guerrillas—trained exclusively for the purpose of warfare and should be distinguished from a sanctioned militia or a levy, which are temporary forces— citizen soldiers with less training, who may be "called up" as a reserve force, when a nation mobilizes for total war, or to defend against invasion. The term military is often used to mean an army.
The doctrine that asserts the primacy of a military within a society is called militarism.
As an adjective, "military" is a descriptive property of things related to soldiers and warfare. It also refers to such context dependent terms such as military reserves which may indicate an actual unit deployable on command or the general sense, of a Nation States reserve troops available to or eligible for duty in its armed forces.
In formal British English, "military" as an adjective refers more particularly to matters relating to an army (land forces), as opposed to the naval and air force matters of the other two services.
In American English, "military" as an adjective is more widely used for regulations pertaining to and between military procurement, military transport, military justice, military strength, and military force.
Military procurement refers to common regulations and requirements for a ship or a detached unit to requisistion and draw on a base's facilies (housing, pay, and rations for detached personnel), supplies (most commonly food stocks or materials, and vehicles) by the service running a primary base; e.g. Army units detached to or staging through an air base, a vessel calling at a port near an army or air base, an army unit drawing supplies from a naval base.
Military transport would pertain to an equipment trans-shipped via a sister service, or an individual detached for a technical school operated by a sister service, or the travel orders and authorization of such an individual to proceed via a sister services vehicles, as well as the drawing (loan of) transportation assets (staff cars, Hum-Vees, military trucks) operating from the primary base command.
Military Justice, as in the Uniform Code of Military Justice. Most nations have a separate code of law which regulates both certain activities allowed only in war, as well as provides a code of law applicable only to a soldier in war (or 'in uniform' during peacetime).
The statutory laws set down by the United States Congress to apply to the individual conduct within any military force of the United States— these are the specific articles under which a soldier or sailor would be tried for infractions ranging from minor (Late Return, petty theft) to severe (Rape, Murder); this code is usually referred to by the acronym UCMJ.
Military strength is a term that describes a quantification or reference to a nation's standing military forces or the capacity for fulfillment of that military's role. For example, the military strength of a given country could be interpreted as the number of individuals in its armed forces, the destructive potential of its arsenal, or both. For example, while China and India maintain the largest armed forces in the world, the U.S. Military is considered to be the world's strongest.
Military Force is a term that might refer to a particular unit, a regiment or gunboat deployed in a particular locale, or as an aggregate of such forces (e.g. "In the Gulf War the United States Central Command controlled military forces (units) of each of the five military services of the United States.").