Artillery
Volley gun is a gun with multiple single-shot barrels that volley fired simultaneously or sequentially in quick succession. Although capable of unleashing intense firepower, volley guns differ from modern machine guns in that they lack autoloading and automatic fire mechanisms
Siege artillery: Large-caliber artillery that have limited mobility with indirect firing trajectory, which was used to bombard targets at long distances.
Large-calibre artillery.
Field artillery: Mobile weapons used to support armies in the field. Subcategories include:
Infantry support guns: Directly support infantry units.
Mountain guns: Lightweight guns that can be disassembled and transported through difficult terrain.
Field guns: Capable of long-range direct fires.
Howitzers: Capable of high-angle fire, they are most often employed for indirect-fire.
Gun-howitzers: Capable of high or low-angle fire with a longer barrel.
Mortars: Typically muzzle-loaded, short-barreled, high-trajectory weapons designed primarily for an indirect-fire role.
Gun-mortars: Typically breech-loaded, capable of high or low-angle fire with a longer barrel.
Tank guns: Large-caliber guns mounted on tanks to provide mobile direct fire.
Anti-tank artillery: Guns, usually mobile, designed primarily for direct fire to destroy armored fighting vehicles with heavy armor.
Anti-tank gun: Guns designed for direct fire to destroy tanks and other armored fighting vehicles.
Anti-aircraft artillery: Guns, usually mobile, designed for attacking aircraft by land and/or at sea. Some guns were suitable for the dual roles of anti-aircraft and anti-tank warfare.
Rocket artillery: Launches rockets or missiles, instead of shot or shell.
Railway gun: Large-caliber weapons that are mounted on, transported by and fired from specially-designed railway wagons.
Naval artillery: Guns mounted on warships to be used either against other naval vessels or to bombard coastal targets in support of ground forces. The crowning achievement of naval artillery was the battleship, but the advent of air power and missiles have rendered this type of artillery largely obsolete. They are typically longer-barreled, low-trajectory, high-velocity weapons designed primarily for a direct-fire role.
Coastal artillery: Fixed-position weapons dedicated to defense of a particular location, usually a coast (for example, the Atlantic Wall in World War II) or harbor. Not needing to be mobile, coastal artillery used to be much larger than equivalent field artillery pieces, giving them longer range and more destructive power. Modern coastal artillery (for example, Russia's "Bereg" system) is often self-propelled, (allowing it to avoid counter-battery fire) and fully integrated, meaning that each battery has all of the support systems that it requires (maintenance, targeting radar, etc.) organic to its unit.
Aircraft artillery: Large-caliber guns mounted on attack aircraft, this is typically found on slow-flying gunships.
Nuclear artillery: Artillery which fires nuclear shells.
Modern field artillery can also be split into two other subcategories: towed and self-propelled. As the name suggests, towed artillery has a prime mover, usually an artillery tractor or truck, to move the piece, crew, and ammunition around. Towed artillery is in some cases equipped with an APU for small displacements. Self-propelled artillery is permanently mounted on a carriage or vehicle with room for the crew and ammunition and is thus capable of moving quickly from one firing position to another, both to support the fluid nature of modern combat and to avoid counter-battery fire. It includes mortar carrier vehicles, many of which allow the mortar to be removed from the vehicle and be used dismounted, potentially in terrain in which the vehicle cannot navigate, or in order to avoid detection.

Organizational types: At the beginning of the modern artillery period, the late 19th century, many armies had three main types of artillery, in some case they were sub-branches within the artillery branch in others they were separate branches or corps. There were also other types excluding the armament fitted to warships:

Horse artillery, first formed as regular units in the late 18th century, with the role of supporting cavalry, they were distinguished by the entire crew being mounted.
Field or "foot" artillery, the main artillery arm of the field army, using either guns, howitzers, or mortars. In World War II this branch again started using rockets and later surface to surface missiles.
Fortress or garrison artillery, operated a nation's fixed defences using guns, howitzers or mortars, either on land or coastal frontiers. Some had deployable elements to provide heavy artillery to the field army. In some nations coast defence artillery was a naval responsibility.
Mountain artillery, a few nations treated mountain artillery as a separate branch, in others it was a speciality in another artillery branch. They used light guns or howitzers, usually designed for pack animal transport and easily broken down into small easily handled loads
Naval artillery, some nations carried pack artillery on some warships, these were used and manhandled by naval (or marine) landing parties. At times, part of a ship's armament would be unshipped and mated to makeshift carriages and limbers for actions ashore, for example during the Second Boer War, during the First World War the guns from the stricken SMS Königsberg formed the main artillery strength of the German forces in East Africa.

After World War I many nations merged these different artillery branches, in some cases keeping some as sub-branches. Naval artillery disappeared apart from that belonging to marines. However, two new branches of artillery emerged during that war and its aftermath, both used specialised guns (and a few rockets) and used direct not indirect fire, in the 1950s and 1960s both started to make extensive use of missiles:

Anti-tank artillery, also under various organisational arrangements but typically either field artillery or a specialist branch and additional elements integral to infantry, etc., units. However, in most armies field and anti-aircraft artillery also had at least a secondary anti-tank role. After World War II anti-tank in Western armies became mostly the responsibility of infantry and armoured branches and ceased to be an artillery matter, with some exceptions.
Anti-aircraft artillery, under various organisational arrangements including being part of artillery, a separate corps, even a separate service or being split between army for the field and air force for home defence. In some cases infantry and the new armoured corps also operated their own integral light anti-aircraft artillery. Home defence anti-aircraft artillery often used fixed as well as mobile mountings. Some anti-aircraft guns could also be used as field or anti-tank artillery, providing they had suitable sights.
However, the general switch by artillery to indirect fire before and during World War I led to a reaction in some armies. The result was accompanying or infantry guns. These were usually small, short range guns, that could be easily man-handled and used mostly for direct fire but some could use indirect fire. Some were operated by the artillery branch but under command of the supported unit. In World War II they were joined by self-propelled assault guns, although other armies adopted infantry or close support tanks in armoured branch units for the same purpose, subsequently tanks generally took on the accompanying role.

Equipment types: The three main types of artillery "gun" are field guns, howitzers, and mortars. During the 20th century, guns and howitzers have steadily merged in artillery use, making a distinction between the terms somewhat meaningless. By the end of the 20th century, true guns with calibers larger than about 60 mm have become very rare in artillery use, the main users being tanks, ships, and a few residual anti-aircraft and coastal guns. The term "cannon" is a United States generic term that includes guns, howitzers, and mortars; it is not used in other English speaking armies.
The traditional definitions differentiated between guns and howitzers in terms of maximum elevation (well less than 45° as opposed to close to or greater than 45°), number of charges (one or more than one charge), and having higher or lower muzzle velocity, sometimes indicated by barrel length. These three criteria give eight possible combinations, of which guns and howitzers are but two. However, modern "howitzers" have higher velocities and longer barrels than the equivalent "guns" of the first half of the 20th century.

True guns are characterized by long range, having a maximum elevation significantly less than 45°, a high muzzle velocity and hence a relatively long barrel, smooth bore (no rifling) and a single charge. The latter often led to fixed ammunition where the projectile is locked to the cartridge case. There is no generally accepted minimum muzzle velocity or barrel length associated with a gun. 
Howitzers can fire at maximum elevations at least close to 45°; elevations up to about 70° are normal for modern howitzers. Howitzers also have a choice of charges, meaning that the same elevation angle of fire will achieve a different range depending on the charge used. They have rifled bores, lower muzzle velocities and shorter barrels than equivalent guns. All this means they can deliver fire with a steep angle of descent. Because of their multi-charge capability, their ammunition is mostly separate loading (the projectile and propellant are loaded separately).
That leaves six combinations of the three criteria, some of which have been termed gun howitzers. A term first used in the 1930s when howitzers with a relatively high maximum muzzle velocities were introduced, it never became widely accepted, most armies electing to widen the definition of "gun" or "howitzer".