Pseudoscorpions are tiny arachnids, members of the order Pseudoscorpiones. They are quite small, from 2 to 10 millimeters in length on average, though one species native to Ascension island grows to 13mm. They have a flat, segmented body that somewhat resembles a louse, with four pairs of legs (as is characteristic of arachnids), and a pair of scorpion-like pedipalps.
The movable portion of their pedipalps has a venom gland which is used in capturing prey. Their venom, however, poses no threat to humans. Like spiders, they are capable of spinning silk, but they don't use it for webbing. Rather, they make disk-like cocoons which they use for shelter from adverse weather.
Pseudoscorpions prey on mites, small ants and other minuscule bugs. Because of this, they are largely beneficial to humans. One species, Chelifer cancroides, is somewhat notorious for popping up in libraries and homes, where it eats booklice and dust mites.