Slag only exists in Borderlands 2 and deals full damage to Flesh, Shielded and Armored Enemies. It is the only element in Borderlands 2 that doesn't deal damage over time.
Slagg Weapons generally have a 1.5 times higher elemental effect chance, than other elements.
Normal / True Vault Hunter Mode: The Slag Debuff lasts for 8 seconds and doubles all non-Slagg damage. (including Melee Damage, other elemental damage, critical hits and damage over time effects)
Ultimate Vault Hunter Mode: The Slag Debuff lasts for 15 seconds and triples all non Slagg damage, while also boosting Slag Damage by 50%.