DPS

DPS is short for damage per second.

Practically speaking, having a rough idea of your unit's Damage Per Second is really important to know what weapons do well against what armour types. You can find the DPS chart here.

DPS already includes such factors as Accuracy and Reload Rate.

How it is calculated
DPS = "(Average Damage * Accuracy) / Reload Rate"
 * Average Damage = "(( Min damage + max damage ) / 2) * (AP / 100)" or "weapon minimum damage" (whichever is higher)
 * Accuracy is a measure of how often a weapon hits. Weapons tend to lose accuracy when being used on the move. Accuracy introduces an element of randomness into the game.

All DPS values on the DPS chart are for standing and firing only.
 * AP is the armor piercing value of the weapon against the armor type in question.
 * Reload Rate is a measure of how often a weapon shoots. A reload rate of 1 shoots once per second. A reload rate of 8 shoots once every 8 seconds.

What is not factored into the DPS Chart
Certain weapons have a lower or higher effective DPS than what is shown on the DPS chart.

Actual damage output is affected by many in-game factors, listed below.

Variance that results from Accuracy
It's certainly possible for a weapon to hit five times in a row, then miss five times in a row. This is very important for weapons with slow rates of fire, such as Snipers. This can distort how strong you feel the weapon is.

Area Effect Damage
For area of effect weapons, this is because they hit multiple targets. Area of Effect has been written about extensively here.

Maximum Range
Shooting with a unit that has a longer maximum range gives you the advantage of firing more shots in a given fight, thus your effective DPS is actually higher.

Abilities
Certain abilities can increase or decrease a unit's DPS. For instance, Guardsmen Sergeants and Commissars both increase the ranged DPS of guardsmen.

Formation
Larger squads have a more difficult time getting into maximum range, as a result of their formations and pathing. This reduces the effective damage output.

Rotation Rates
Rotating a unit can cause it to not shoot, reducing your effective DPS. This happens to units such as XV88 Broadsides that rotate slowly.

Firing Angle
Certain units can only fire in a narrow cone. This most often happens when firing on the move with infantry that are chasing others.

Researches
Every race can increase their DPS via researches. Certain units scale in strength at a greater rate than others, which is not shown by the DPS Chart. A good example of this are Eldar Rangers.

Squad Sizes
The DPS chart shows the damage of one weapon against one armour type. Larger squads have more weapons.

Multiple Weapons
Certain units can use multiple weapons simultaneously. A great example of this is the Hellfire Dreadnought, which has both a Missile Launcher and lategame Lascannon.

Cover
Armies with strong melee and ranged are more able to take and hold cover, which means that they win ranged firefights better than their raw ranged DPS would otherwise imply. This is important for Chaos.

Mass
A unit with lower mass is more easily disrupted by weapons that deal force damage, such as Artillery, Grenade Launchers or Mines. This reduces their effective DPS when such weapons are employed. This is most important for Eldar and Sisters of Battle.