USBs are random drops that contain Weapon Skins. They require USB Keys to open, which can only be purchased from the store for $2.49 or bought on the Steam Marketplace.
Unwanted USBs can either be sold on the Marketplace, or recycled into Weapon Skin Materials for Crafting.
Stories of War
Horzine First Encounter
Weapon Collector Pack 1
Vault Crates (which only drop from the Vault) can also contain Weapon Skins, and unlike normal crates or usbs, Vault Crates are free to open without needing to buy a key. Vault Crates have their own set of weapon skins exclusive to them.
Console versions do not use USBs, and instead have Crates for both Gear and Weapon Skins.