Since this is often asked, I think it would be useful to explain what all the mods & artifacts do. If you have more info on existing mods or artifacts, new ones or corrections to do, please comment below.

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WEAPON MODS :

Ammo maximum : Enables you to carry more ammo for the corresponding weapon.
Armour piercing damage : Makes you deal additional armour piercing damage that is particularly efficient vs heavy armored targets (like Grineer lancers).
Charge damage : Increases the damage of your charged melee strikes.
Clip capacity : Makes you clips carry more bullets.
Critical chance : Adds chances that you will hit a critical strike (stronger attack displayed in yellow)
Critical damage : Makes your critical strikes deal even more damage (the base multiplier being 1.5).
Damage : Makes you deal additional damage.
Electricity damage : Makes you deal additional electricity damage that is particularly efficient vs robotics and stuns most non-shielded targets.
Fire damage : Makes you deal additional fire damage that is particularly efficient vs light armored targets (like Infestation) and sometimes make enemies panic.
Fire rate : Increases your fire rate (or attack speed on melee weapons).
Freeze damage : Makes you deal additional freeze damage is particularly efficient vs shields and slows enemies.
Heavy Charge speed : Increases the charge speed of your charged melee strikes.
Multishot : Makes you occasionnally shoot multiple bullets, or increases the amount of pellets shot with shotguns. Decimal values represent the percentage of shots that will feature an extra bullet.
Puncture : Makes you bullets go through walls, objects or enemies. The value represents the distance (in meters) the bullet(s) can go through.
Reload speed : Makes you reload your weapon faster.
Stun chance : Gives your attacks a chance to stun your target.

WARFRAME MODS :

Armor : Deacreases the damage you sustain when your shields are down.
Enemy radar : Enemies are shown on your radar. The value represents the distance (in meters) at which your radar is effective.
Health : Increases your maximum health.
Loot radar : Loot is shown on your radar. The value represents the distance (in meters) at which your radar is effective.
Melee damage : Increases the damage dealt by your melee strikes.
Power duration : Increases the duration of your powers.
Power efficiency : Decreases the energy costs of your powers.
Power max : Increases your maximum energy.
Power range : Increases the range of your powers.
Power strength : Increases the damage of your powers.
Shield capacity : Increases your maximum shield capacity.
Shield recharge : Increases the rate at which you shield recharges.
Sprint speed : Increases your speed while sprinting.
Stamina Max : Increases your maximum stamina (for running, etc…).
Stamina recharge rate : increases the rate at which your stamina recharges.

On a side note :- Mod values are calculated multiplicatively for the moment, not additively. This means per example that if you have 2 +50% shield capacity mods, you will have a total of 225% shields, not 200%. Health, energy and shields mod stack additively.

ARTIFACTS :

Artifacts provide bonuses for all your squad. They are stackable. Each player can choose one at the beginning of every mission (these are not 1-mission only consumables, once you got one it stays in your deck). They can be found in some alert missions (about 30% of them) as a mission completion reward (you don’t have to actually find them, they are just awarded at the mission ending). If an alert gives you an artifact as a reward, it will be listed in the rewards list before starting the mission as a “random reward”.

Affinity Amp : Increases the affinity you gain from kills.
Dead Eye : Increases the damage you deal with snipers.
Electrical Resistance : Decreases the elecricity damage you sustain.
Enemy Radar : Shows enemies as red dots on your radar.
Energy Siphon : Passively regenerates your energy.
Fire Resistance : Decreases the fire damage you sustain.
Frost Insulation : Decreases the freeze damage you sustain.
Laser Deflection : Makes you deflect lasers damage.
Loot Detector : Shows loot crates/containers/corpses on your radar.
Physique : Increases you maximum health.
Pistol Amp : Increases the damage you deal with pistols.
Pistol Scavenger : Enemies/crates/containers/corpses drop more pistol ammo.
Rejuvenation : Passively regenerates your health.
Rifle Amp : Increases the damage you deal with rifles.
Rifle Scavenger : Enemies/crates/containers/corpses drop more rifle ammo.
Shotgun Amp : Increases the damage you deal with shotguns.
Shotgun Scavenger : Enemies/crates/containers/corpses drop more shotgun ammo.
Sniper Scavenger : Enemies/crates/containers/corpses drop more sniper ammo.
Sprint Boost : Increases your speed while sprinting.
Steel Charge : Increases your melee damage.
Toxin Resistance : Decreases the poison damage you sustain.

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Additional Guide Tips Welcomed!