Very advanced, fragile aliens shaped like cats o' nine tails that, according to Ringworld, probably evolved on a cold, low gravity world resembling Neptune's moon Nereid. They mostly live on big ships, crossing the interstellar space at sublight velocities (according to A Gift From Earth, they find hyperspace vulgar), trading in information and technology. They possess inertial dampers and planetary drives.
Outsiders are estimated to be the most advanced species in Known Space, possibly the Galaxy, but the extent of their development remains unknown. Though they have the technology to produce advanced faster-than-light drives, they rarely use them, preferring to travel the "slow" way, just below the speed of light. They do possess a "reactionless drive" technology that allows them to reach this speed almost instantaneously.
It was the Outsiders that sold humans the FTL drive, on a stop in the solar system of the human colony called We Made It.
They spend all of their time following starseeds and acting as information brokers to space-faring sentient races throughout the Milky Way. Their prices can be very high and scaled to the estimated impact the information will have on the civilization of the client race. Their most common wares are interstellar propulsion systems of various types. The Outsiders maintain a strict commercial ethos regarding any form of knowledge, which shrouds them in a secrecy only wealth can penetrate. They do not haggle. They will answer any question, even those about themselves, if you are willing to pay the price. Personal questions about the Outsiders have been priced beyond the ability of any individual or government to pay (on the order of a trillion credits).
The starseeds are gigantic, non-sentient space-dwelling animals that travel from the galactic core to the rim by their solar sails. At the rim they lay their eggs, then travel the 50,000 light-years back to the core. In "Peace and Freedom", it is revealed that starseeds are in fact packages of microorganisms designed to seed new planets with life, thus creating new customers for the Outsiders. They are reluctant to reveal this information because they are ashamed, since one of their starseeds created the Thrint, which destroyed nearly all intelligent life in the galaxy several billion years ago.
The story told in "A Darker Geometry" that the Outsiders were created by a race of extra-dimensional aliens seeking to escape the heat death of their own universe is considered as non-canonical and thus a false tale spred out by somebody.