The elements of Pandora interact with surfaces differently, which is relevant in that there are elemental bullets, and any given creature is surely made of surfaces. Just as fire burns harshest against flesh, so fleshy enemies like bullymongs, varkids, and unarmored psychos will take extra damage from Fire weapons. Corrosive damage eats away armor very effectively. It will avail you quite well against the numerous loaders you’ll face, spawned from Constructor Bots or launched during moonshot barrages. Against shielded and cloaking enemies, nothing competes with Shock weaponry. Last but not least, Slag doubles the damage of anything that isn’t Slag, once it’s applied.
Certain element and surface combinations result in damage resistance, too: shields diminish the damage of anything except Shock, flesh mildly diminishes the damage of Corrosive, armor greatly diminishes the damage of Fire. If you aren’t matching up the weapon you’re using with the elemental weakness of your current target, then you’ll at least want to make sure you aren’t using something less effective than normal, either. Details on Normal Mode and True Vault Hunter Mode can be found in both Claptrap’s Guide to Minioning and the Marcus Munitions Monthly sections of this book.
The status effects that elemental attacks can cause—Fire burning, Shock sparking, Corrosive corroding—are by no means guaranteed. Each round or attack will have a chance to successfully cause the effect, which is further skewed by the levels of you and the target—the higher their level, the more likely they resist. Some monsters have a particularly high resistance to a given element, and will ignore that damage most or all of the time. In these cases you’ll simply see “RESIST” popping out about them, and you should switch to a different element posthaste. Often you won’t have difficulty sticking status effects after a few rounds, though, and some skills make the process even more foolproof. And, while only one visual effect will be represented (like green for corrosion or purple for Slag), every effect can be present and dealing damage at once!
Some enemies use physical shielding so thick it cannot be pierced by conventional weaponry; it’s here, among other places, that Explosive rounds shine. Especially the gyrojet-equipped Torgue weapons that fire mini rockets quickly. The blast radius damage from Explosive weapons pierces certain types of shielding where no other weapon will.