Because there seems to be some confusion over the "gazillions" of weapons available in Borderlands, IGN has conjured up a quick article that breaks down the different weapon types and their associated statistics.
Every gun type in the game comes with base statistics, from magazine capacity to accuracy, fire rate, and damage. These all differ between models, and a few other variations can further diversify weapons function. Some have scopes, for instance, and some don't. Many have bonuses depending on their rarity, including bonus critical hit damage, reload speed, ammunition regeneration, and recoil reduction. Many also come with elemental effects, which come in varying strengths. Fire elemental weapons are more effective against flesh whereas shock elemental effects quickly take down shields. Corrosive bullets can cause more damage over time, and the explosive weapons cause enemies to, unsurprisingly, explode. As if that wasn't enough, the even rarer weapon types have unique special effects, such as split-second reloads, arcing rocket shots, and bullets that fire in seemingly magical waves and spirals.
Just to go over a few examples, say you come across a revolver with high damage numbers, a slow rate of fire, a weapon zoom, and explosive elemental effects. Since you'll eventually open up four weapon slots, should you decide to equip that pistol, or instead stay with the SMG that does x3 corrosive damage and has a lighting fast reload. Or maybe you're sick of the more long range combat and want to ditch that ultra high damage sniper rifle with x4 explosive damage and a 150 percent critical hit damage bonus for something more up close and personal. Maybe that Caustic Shotgun with the extended magazine size and corrosive elemental damage might be just what you're looking for.