Military history
Military history is composed of the events in the history of humanity that fall within the category of conflict. This may range from a melee between two tribes to conflicts between proper militaries to a world war affecting the majority of the human population. Military historians record (in writing or otherwise) the events of military history.
Military activity has been a constant process over thousands of years. However, there is little agreement about when it began (Otterbein 2004). Some believe it has always been with us; others stress the lack of clear evidence for it in our prehistoric past, and the fact that many peaceful, non-military societies have and still do exist (See Otterbein, Fry and Kelly in bibliography below).
The essential tactics, strategy, and goals of military operations have been unchanging throughout the past 5,000 years of our 90,000-year human history. As an example one notable maneuver is the double envelopment, considered to be the consummate military maneuver, executed by Hannibal at the Battle of Cannae in 216 BC, over 2,200 years ago. This maneuver was also later effectively used by Khalid ibn al-Walid at the Battle of Walaja in 633 AD, and was earlier described by the Chinese military theorist Sun Tzu, who wrote at roughly the same time as the founding of Rome. By the study of history, the military seeks to not repeat past mistakes, and improve upon its current performance by instilling an ability in commanders to perceive historical parallels during battle, so as to capitalize on the lessons learned. The main areas military history includes are the history of wars, battles, and combats, history of the military art, and history of each specific military service.
There are a number of ways to categorize warfare. One categorization is conventional versus unconventional, where conventional warfare involves well-identified, armed forces fighting one another in a relatively open and straightforward way without weapons of mass destruction. "Unconventional" refers to other types of war which can involve raiding, guerrilla, insurgency, and terrorist tactics or alternatively can include nuclear, chemical, or biological warfare.
All of these categories usually fall into one of two broader categories: High intensity and low intensity warfare. High intensity warfare is between two superpowers or large countries fighting for political reasons. Low intensity warfare involves counterinsurgency, guerilla warfare and specialized types of troops fighting revolutionaries.
Source : http://en.wikipedia.org
Gun
A gun is a common name given to an object that fires high-velocity projectiles. The projectile is fired through a hollow tube known as the gun's barrel. The projectile's caliber, or diameter, is usually designated in fractions of an inch or in millimeters. Differing from the musket, most modern guns are rifled, with a series of grooves spiraling along the barrel; exceptions include smoothbores on tanks, AFVs and some artillery.
The term "gun" is often used synonymously with firearm, but this is common only for civilian usage. In military usage, the term refers only to artillery that fires projectiles at high velocity, such as naval guns (which are never referred to as cannon) or tank guns. A gunner is a member of the team charged with the task of operating and firing a gun. By military terms, mortars and all hand-held firearms are excluded from the definition of guns. The exception to this is the shotgun, which is hand-held, has a smooth bore and fires a load of shot or a single projectile known as a slug.
The word "gun" is also applied to some more or less vaguely gun-like or gun-shaped tools, such as staple guns and glue guns.
In a gun-type fission weapon the "gun" is part of a nuclear weapon. The "projectile" is fissile material that is fired and captured inside the device. In the case of nuclear artillery it should not be confused with the gun that fires the whole warhead.
At times, the word gun is used to describe the person holding the weapon rather than the weapon itself, as in "a hired gun".
Source : http://en.wikipedia.org