In the military sense, the term 'rifle' technically refers to any projectile weapon possessed of a rifled barrel - the term originated in the
1500s and was coined to distinguish such weapons from the smoothbore cannon and
muskets which they gradually replaced. When introduced, rifles were more accurate than their smoothbore cousins on account of the spin the
rifling imparted on the projectile; however, it took almost two centuries for the smoothbore firearm to be completely supplanted, as rifles were more expensive and harder to make, had a lower rate of fire, and required more maintenance. Nonetheless, by the time of the American
War of Independence rifles were common, and the greater accuracy of the rifle, coupled with new and improved powder charges, led to a new school of arms; sniping, a sport which the scurrilous Americans used to great and cowardly effect against Britain's mighty
redcoats. This innovation was followed eventually by breechloading, repeating rifles,
smokeless powder, automatic fire, small-calibre ammunition, polymer construction and improved sights, and thus it is possible to draw a direct from from early rifles - such the
Baker and the later
Brunswick models - to the present day.
Over the years, the precise definition of the word 'rifle' has been quite fluid; nowadays it is used in combination with other words to distinguish the function and general characteristics of specific firearms - at various times it has been qualified to become 'Battle Rifle', 'Long Rifle', 'Assault Rifle', 'Sniper Rifle' and so forth. Most countries define a rifle as a powder firearm with a rifled barrel over a certain length; pistols and submachineguns are therefore not rifles, despite generally being rifled. Neither are shotguns, as these are, with the exception of those designed to fire solid slugs, smoothbore.
Nonetheless a 'rifle' is assumed to be a lengthy shoulder arm, with a manual action, accurately firing a high-powered round from either an integral magazine, clips, or a removable box (most commonly the former). Whilst technically this definition can be applied equally to a bolt action Lee Enfield Mk IV, a lever action Marlin 1895, and a modern Heckler and Koch G36, only the former two would be referred to by the man on the Clapham omnibus as rifles, unqualified; perhaps it's the wood.
Between the 1700s and the third quarter of the 20th century the rifle was the principal arm of modern armies; after sterling service in 19th century colonial wars against spear-armed tribesmen, the rifle's inadequacy against modern opponents was thrown into relief by World War One, during which it became apparent that long-range destruction in modern war was best carried out by artillery, machine-guns, tanks, bombers and high explosives. For the task of assaulting trenches, the long, heavy, bolt-action rifle was less useful than a sharpened entrenching tool and a bag of hand grenades. Subsequent developments in the history of the firearm led to smaller, semi-automatic, automatic weapons designed to spit large quantities of ammunition from the shoulder and the hip whilst on the move; the assault rifle. By the end of WW2 the long rifle was effectively obsolete in front-line service, and was relegated to a training role; the last Western army to retain the rifle as the mainstay of the regular forces was that of Italy, who used a derivative of the M1 Garand until the early 80s.
On a larger scale than the infantry arm, artillery is usually rifled as well, whilst portable anti-tank weapons and tank cannon are more often smoothbore; both the British Challenger 2 and the the America M1 Abrams battle tanks (amongst others) are equipped with 120mm smoothbore cannon, the advantage being that such a cannon can fired shaped charge warheads such as the venerable HEAT round.