"There has been much debate among the Horadrim regarding the shadow vermin. Some scholars believe that these wraithlike foes are formed from concentrated demonic energy, while others believe they are the souls of humans corrupted by Diablo. They glide through the darkness as easily as water, ready to smother their unwary prey."— Deckard Cain(src)
Visually, they resemble pitch-black pools of smoking liquid that stick to the ground, with humanoid forms emerging when they attack.
Shadow Vermin are the most numerous and weakest of all enemies encountered in Acts II, III, and IV. The apotheosis of their swarming rush is the battle with Iskatu at the beginning of Act IV. While they have no special abilities and attacks, they are quite fast, and their melee blows, while weak, are delivered at fast rate, and due to the appearance of these demons it may be challenging to see them in time (and even determine how many are you facing at the moment once encountered).
They are encountered for the first time in the Realm of Shadow in Act II.
In Shrouded Moors and Temple of the Firstborn, one may encounter identical, though slightly red-tinted, Blood Vermin. Their most notable appearance is during the Blood Statue / Ascension Ritual events and fight with Vidian.