Unown are floating, flat, eye-shaped Pokémon with appendages that make them resemble letters. They are black all over. All alphabetic Unown have fully open eyes while the two punctuation Unown have half-closed eyes.
Unown has 28 different forms, one for each letter of the alphabet, as well as a form for an exclamation point (!) and a question mark (?). These are chronicled in the Unown Dex in Generation II and the Unown Report in HeartGold and SoulSilver.
The exclamation point and question mark forms were not introduced until Generation III.
There are also many glitch forms.
Unown's only known move in battle is the enigmatic Hidden Power.
In the anime, they are capable of reading the thoughts of other lifeforms. Great numbers of them are capable of warping reality to suit the dreams and wishes of whoever they read the thoughts of and is in control of the Unown.
Their mind reading ability seems to suggest that they should be able to learn Mind Reader. However, they can only do this when more Unown are present, so it could be a shared effort rather than an ability unique to each Unown.
Unown can make telepathic contact with humans (as well as many other creatures). Unown have a timidness that makes them rarely venture outside of their dimension. They tend to stick to walls or orbit other Unown when in a group.
An Unown fell out of the Unown dimension in Address Unown!. After it was healed by the group, it helped defend against Team Rocket and its mind merged with that of Ash's Larvitar so that the group could help it to address its inner pain.