Artillery guns are the backbone of the Infantry Support Divisions, they are what tanks are to infantry, a saving grace. Artillery guns started all the way back to when the concept of trebuchet's and catapults made its rise, the meaning behind artillery is to punt objects a large distance, hence the invention of the trebuchet and the catapult.
Artillery guns took heavy shifts in WWI when Germany used them heavily in the invasion of Paris, a notable gun that was used during the raids was the Paris Gun, which was an extremely long gun firing 210mm shells, this gun was remarkably effective during the assaults, it was able to punt shells over 75 miles away from it, firing 20 shells per day. Another notable weapon was the Big Bertha Howitzer, which garnered a strong reputation of being a remarkably effective weapon, being referred to the German Press as a "Wonder weapon".
There are many classes of artillery, first off the mortar weapons, which is really a miniature howitzer that punts 75mm shells a short distance, this can be defined as a close range support weapon, these weapons saw extensive service during world war II when the need for close range support was valuable to battles, these weapons have an advantage by being an incredibly light support platform, only needing 2-4 people operating the gun, some mortars were mounted on vehicles by Nazi Germany to give light self-propelled artillery guns, but these were mainly used in recon.
Infantry Support Guns, these are very similar to mortar guns, but they have a significantly higher calibre, almost heading into the 15cm category. These weapons have a reputation as a scary platform to have to go against in the heat of battle, mainly cause of its high calibre, but its not walk in the park to operate, namely because it is a heavy module one has to operate, really relying on a vehicle to get them from place to place, and with the huge recoil, this gun became increasingly difficult to mount on early Nazi German tanks, the tanks that did possess this type of artillery was the Grille SPG, and the famous Brummbar.
Anti-Tank guns are considered an artillery piece because it to punt's shells at long ranges, but this type of artillery is reserved mainly for tanks and other AFV's, An anti-tank has the howitzer setup, with two stands to absorb the recoil, and the two wheels to get it from one place to another, but these weapons fire armor piercing rounds instead of the standard HE Projectile that howitzer's use in the heat of battle, the most prominent Anti-Tank guns that have been used are the Pak 43 AT Gun, and the 17 Pounder Anti-Tank gun.
Rocket Artillery is a relatively new concept that most people don't know about, when people think Artillery, they think a single barreled gun that punts one shell at time, at a slow rate, and for the most part that's true, but rockets can be used as artillery, able to create a wall of rockets that no one can out run from. Rocket artillery cannot reload on the spot, so a lot of shoot-and-scoot tactics are involved with this vehicle, this concept of rocket artillery was officially started in WWII when the Germans introduced the nebelwerfer, also known as the "Screaming Meemie", but took its major shift in the 70's when the M270 MLRS was developed.
Howitzer's are the single barreled artillery guns that has been in almost every war during and after WWI, these guns operate in batteries of 4 and up and operate according to soldiers who in the field call them to fire, many armies use these as fire support weapons to help turn the tide of the battle very often, and are often used to assault forts, and dug-in positions, these guns are often placed on mountains to give them a very clear view of the battlefield. Notable howitzers that are in use today is the M198 Howitzer's, and the M777 Lightweight Howitzer.
Anti-Aircraft Guns are artillery guns because they to punt shells a great distance, but the distance is used to take down airplanes, these guns also operate batteries of 4-6, or sometimes alone, with a flood light to add, these guns were used heavily in WWII. Batteries of 4 were often used with super-heavy or heavy AA guns, lighter guns were used alone, like the Bofors 40mm AA gun. A definitely effective AA gun that was powerful and versatile, was the famous Flak 88 which was often found in every battlefield in WWII, used to take down both aircraft and it was able to take down any allied tank.
Super-Heavy/Railway Guns, these are very expensive and very immense guns, these heavy guns are used when assaulting a heavily fortified building, these have to be deployed near a railway, limiting there deployment anywhere. These guns had to debate between to things, either have the longest range, or highest calibre, either design often hampered the weight, making this class a very heavy one. Whilst limited in there role, the role they sometime got to play were scary and somewhat devastating when seen up close, guns such as the Schwerer Gustav had an 800mm calibre round that could be punted 37,210m away from the target, and with these heavy, immense shells, this gun was a big hit and was often scary when the allied troops had to face it.