This is a general guide for what each artillery piece is in conquest. Most if not all of these also apply to skirmish as well. (Apologies for the lazy line-work but I could not care less)
NOTE; These are not necessarily real terms the game uses or actual guns are referred to as, this is simply how I categorize them based on use. To know exactly how each individual gun works, test them out in the editor to see what you like.
Artillery generally fall into 3 types of firing arcs; Direct fire, arc fire, high arc fire. They also tend to have sizes listed (light, medium/no-size listed, heavy). The smaller the gun, the faster it usually fires and the more accurate it is (unless direct fire), but smaller explosion and less range.
Direct fire is most common with things like Anti-Tank Guns. These guns do best when used as anti-vehicle weaponry, however they can do okay in open fields, against buildings, or when aimed downhill.
This includes: Anti-Tank Guns (self explanatory, use these to kill tanks and armored vehicles), the Recoilless Rifle and the Finnish two-man Anti-Tank Rifle, AA Guns, and some Field Guns.
Arc fire is overall the most common type of fire, and most of your artillery will use it. The arc varies depending on what type of gun it is, but generally you need a line of sight or to be placed far enough back from the thing you’re shooting over.
Arc fire guns include: Infantry Guns/Some Field Guns (insanely good anti-infantry weapons, may be able to deal with lightly armored vehicles as well), Field Howitzers (in-between high arc fire and arc fire, good for hitting targets at a long distance), Rockets, and non-tracked versions of Artillery (in-game classification, very useful I know. Basically the in-between of a howitzer and field gun).
High arc fire is basically only mortars. They can fire over just about anything, but tend to be pretty inaccurate.
This includes: Mortars, Siege Mortars, Ampulomet Molotov Launcher, and arguably Howitzers and tracked Heavy Artillery.