This sounds so simple. Weapons cause damage on other units. Basically, the damage value is deducted of the hit units health. When the health reaches zero the unit ist destroyed.
This page tries to bring some light in the values which are displayed in this wiki and how they can be used to calculate or understand how much damage one unit actually causes on another unit.
Anyway, the topic is complicated. And therefore this page is incomplete and will contain errors.
The Value "Damage" in the Game and the Wiki Tables
What you see as "Damage" in the game and the various Wiki tables (unless marked otherwise) is actually the basic "Damage per second" (short "dps") value of a units weapon. While this is a useful value to compare different units it might be a bit misleading when you observe the actual damage done by a unit.
Actually units/turrets have different shot intervals or weapon shoots salvos consisting of multiple shots with intermittent pauses. A unit which shoots once per 2 seconds will actually do double the damage per shot as indicated in the "Damage" value. A unit which shoots 2 times per second will do half the damage per shot as seen in "Damage".
There's one exception: Starships (should) show the actual damage caused by one run in the "damage" field.
Units often inflict different Damage on different kinds of target units. This is reflected by the percentages listed in the Multiplicator (or Modifier) table. These values are normally hidden in the game and only can only sometimes be guessed by the units descriptions.
To get the actual damage you take the multiplier and multiply it with the damage value.
Example: A lv X Anti-Vehicle Skiff does a basic value of 2,310 dps.
If the Skiff attacks a wall the table shows a multiplier of 75%. 2,310 x 75% = 1733, so a damage of 1733 is inflicted on a wall.
If the Skiff attacks a Bruiser Vehicle the table shows a multiplier of 200%. 2,310 x 200% = 4620, so a damage (per second) of 4620 is inflicted on i.e. an AT-AT.
Splash damage is usually a term of multiple percentage values. The first value applies to targets within 1 Tile of the hit zone. The second value applies to targets within 2 tiles from the hit zone and so on. The splash area is not circular it is a square. So, roughly like this:
So with splash damage the final damage calculates as: (unit basic damage) x (multiplier on target) x (splash damage)
There are still uncertainities. I read somewhere a dev post that units (or starships only) may inflict 100% damage on the center even if the splash damage reads a lower multiplicator.
Buff / Nerfs / Special Abilities
Finally, there are Buffs and negative Buffs = Nerfs. These are perhaps best known from the Factory Outposts in Squad Wars and activated Armory Equipment. Buffs are usually additional multipliers for offensive and defensive values which will have to be discussed on another page.
Many units also have secondary attacks or special abilities which are not reflected in the basic "damage" value. Each of these abilities/attacks can have an own multiplier table. It is quite complicated to reflect these values correctly on the units pages. So it may still take a bit of time until they are completely listed.