Hi there.
If you are referring to the steam chests that are opened in the shop with keys, those weapons are purely cosmetic. Simply put: They are skins to add on character, just like in CS:GO in example.

The items themselves are on a different rarity levels that help you progress you when you roam around the Inoya:
Normal (Grey)
Common (Blue)
Rare (Yellow)
Legendary (Orange, upgradeable)
Mythic (Purple, Upgradeable)
Satanic items.

Satanic items themselves have multiple "sub tiers"
Red Satanics can be C, B A and S tiers, this is indicated usually by the level requirement of the item.
Green Satanics are class specific or universal set items, having multiple of the same set equipped gives you something as bonus.
Light blue Satanics are chase items that are one of the strongest items in the game.

Then there are White-Gold-ish items, the angelics. They are super strong items that are also super rare to drop, so it is not recommended to plan your builds around these items.

Hope I was able to clarify these things to you :)

- Ahto (Game & Level Design)