According to Gearbox, there are over 17,750,000 different variations of weapons, as of release. The game uses a procedural process to generate its various guns in certain classes, such as handguns, shotguns, assault rifles, sniper rifles, and more, but with many variations of firing speed, reload speed, damage type and more.
Thankfully, there is an easy way to determine the basics of a weapon without having to pick it up and open an inventory screen. When a weapon on the ground is approached, a small holographic window appears above it that displays information such as weapon type, damage type, monetary value, as well as the weapon's rarity using a color coding system.
If a human enemy is carrying a special weapon that they drop upon their death, they will wield it in combat. Otherwise they use a default weapon for their enemy type.