As described in the novel, Dwellers are extremely long-lived inhabitants of gas giant planets (like Jupiter, although that particular planet is described in the book as not having enough water for them).
Immature Dwellers are described as anorexic manta rays and stay in this form for about a century. Adult Dwellers are described as consisting of two discs, similar to a yo-yo, with various appendages at edges and hubs including two long spindle arms. They are large in adulthood, with small examples being about five meters in diameter and larger examples ranging up to ten meters. They are neutrally buoyant in a gas giant's atmosphere, and move by rotation of their disks, called "roting".
Their long lifespan (individuals can live for billions of years, the species has existed for ten billion) has made them anarchic and wise. They use the idea of kudos to define their sense of value within a society, and will trade or bet kudos like money. They claim that they have existed since the "First Diaspora", roughly two and a half billion years after the creation of the universe.
Dwellers are male for 90% of the lifespan, turning briefly female in order to have children. Dwellers do not care for their children after birth; the children are often taken into slavery or hunted down as game. Aborted children are kept as pets.
They pass their time in intrigues and ritualized 'war', giving the superficial impression to some that they are less than formidable, although it is widely noted that species who interfere with them tend to suffer catastrophic consequences, sometimes millennia later. Over the course of The Algebraist, they show that despite their apparently haphazard attitude to weaponry and other high-tech artefacts, they do maintain exceptionally powerful weapons to defend their gas giants. At one point, when they are threatened by a formidable alien fleet, they deploy a vast spherical weapon which virtually annihilates the enemy with a single shot.