Artillery is a term which is used to refer both to heavy weapons designed to launch projectiles across fairly long distances, and to the crews who run such weapons. People also use the term more generally to refer to the study of guns, known as gunnery. Artillery of some form or another has been used in warfare for centuries, and often it is at the cutting edge of military technology; in the 20th century especially, artillery became especially important.
Several things set artillery aside from other types of weapons. In the first place, an artillery weapon typically requires a special housing, and the efforts of several people are needed to operate the weapon. The weapon is also designed to shoot extremely large caliber projectiles, which can deal significant damage when they land. Additionally, crews use indirect fire techniques to operate their weapons, which means that instead of visually acquiring a target and aiming at it, the crew uses mathematical formulas to establish a range to the enemy to fire the weapon across a greater distance.
Thanks to indirect fire, artillery can be used even when visibility is poor, as long as soldiers on the ground have some idea as to where the enemy is. It is common for artillery to fire a short opening salvo to find the range, and then adjust after this salvo as needed. Sometimes, a short salvo is all that is required, especially with shells that detonate explosively, flinging shrapnel across a wide area. An artillery emplacement is known as a battery.
Artillery can be fixed, as is the case with coastal and anti-aircraft batteries, or it can be mobile, as with field artillery. Fixed weapons are often extremely heavy, and many are optimized for specific ranges so that they can be used effectively for defense of airfields, coastlines, and so forth. Field artillery is more lightweight, but it still typically requires the use of a heavy truck to be moved.
Most militaries have an artillery division, typically within the army, and members of the artillery may choose to focus on specific theoretical situations or weapons in their training. Modern weapons are quite sophisticated, doing much of the math for their operators, and allowing people to input basic information about range and known weather conditions in order to optimize their operation. There are all sorts of uses for artillery, ranging from blanketing an area with fire before advancing ground troops to shooting enemy aircraft out of the sky.