Tech

Tech are equippable/upgradable bonuses that you attach to your weapon, or find on an existing weapon already.  

Below is a table that breaks down techs and their effects:

TechEffect
Overdrive+Damage, -Recoil
Rapid+Reloadspeed, -Recoil
Stabilizer+Recoil, -Spread
Ventilation+Spread, -Noise
Augment+Movespeed, -Reloadspeed
Localising+Spread, -Penetration

Keep in mind that +Spread doesn't mean that the spread gets worse, the same way that -recoil does not mean that the recoil gets more manageable.  
These +'s and -'s just refer to the way it affects the stat readout.  

The rarity of the tech impacts the stats it gives.  
Common will be +1 or -1, Mythic will be +7 or -7, respectively.  
Techs can also be upgraded by fusing 3 of the same rarity together until mythic, similar to items and armors.

From mythic you need a tech upgrade kit to upgrade tech further!  
(Tech upgrade kit can be crafted with Oil from world boss geoff, diamond and a stat mutator)  
Tech upgrade kit upgrades are 100% unobtainium, 60% ascended, 40% abyssal and 33% void.  
This means once it's unobtainium it will fail several times before getting to the higher tier.

You can also upgrade locked tech by going to the Modify Tech window and clicking the arrow in the corner of the tech.

Tiers

Tier 2, 3, 4, 5 boost the weapon stats by 1, 2 and 3% respectively! If you're lucky enough you can find a Tierifier. These can be found in Epic Expert or higher!  
Boss weaponry boost the weapon stats by a significant amount too and come with their own tech! These can be found in Elite raids or higher

Weapon Mods

Weapon mods can be obtained through the auction, crates that can drop them(these mention on highlight), the "Mod Hunt" quest, or beating bosses in Epic Elite and higher!  
The mod you get is random with a random %, once you have the mod you want you can use more weapon mods to upgrade your current weapon mod by 1 Level up to 10/15 for different mods.  
Fire rate being a universal stat of all the weapon mods letting you pic and mix more with other mods to fit your play style.

ModChange
Weight StabilizerIncreases your handling on your weapon.    
Precision PinReduces condition damage.
Magnetized MagazineIncreases reload speed.
Plastic ReplicaIncreases movement speed.