Acid is a green Liquid that harms the player and some mobs. The player can avoid harm by having a immunity to it like an equip piece encrusted with Emerald. Acid also has the ability to break background blocks such as Trees, Bushes, Grass, etc..
This is a good liquid for mob traps, because most mobs are harmed, it is a versatile liquid. Items do not burn when dropped into Acid (unlike Lava).