(from:
http://en.wikipedia.org/wiki/The_Culture )
Living space
Much of the Culture's population lives on Orbitals, vast artificial worlds that can accommodate billions of people. Others travel the galaxy in huge space ships such as GSVs ('General Systems Vehicles') that can accommodate hundreds of millions of people. Almost no Culture citizens are described as living on planets, except when visiting other civilisations. The reason for this is partly because the Culture believes in containing its own expansion to self-constructed habitats, instead of colonising or conquering new planets. With the resources of the universe allowing permanent expansion (at least assuming non-exponential growth), this frees them from having to compete for living space.
The Culture, and other civilisations in Banks' universe, are described as living in these various, often constructed habitats:
Airspheres
These are vast, brown dwarf-sized bubbles of atmosphere enclosed by force fields, and (presumably) set up by an ancient advanced race at least one and a half billion years ago. There is only minimal gravity within an airsphere. They are illuminated by moon-sized orbiting planetoids that emit enormous light beams.
Citizens of the Culture live there only very occasionally as guests, usually to study the complex ecosystem of the airspheres and the dominant life-forms: the "dirigible behemothaurs" and "gigalithine lenticular entities," which may be described as inscrutable, ancient intelligences looking similar to a cross between gigantic blimps and whales. The airspheres slowly migrate around the galaxy, taking anywhere from 50 to 100 million years to complete one circuit. In the novels no one knows who created the airspheres or why, but it is presumed that whoever did has long since sublimed but may maintain some obscure link with the behemothaurs and lenticular entities. Guests in the airspheres are not allowed to use any force-field technology, though no reason has been offered for this prohibition.
The airspheres resemble in some respects the orbit-sized ring of breathable atmosphere created by Larry Niven in "The Integral Trees", but spherical not toroidal, require a force field to retain their integrity, and arose by artificial rather than natural processes.
Orbitals
Main article: Orbital (The Culture)
One of the main types of habitats of the Culture, an orbital is a ring structure orbiting a star as would a planet. Unlike a Ringworld or a Dyson Sphere, an orbital does not enclose the star (being much too small). However, like a ringworld, the orbital rotates to provide an analog of gravity on the inner surface. A Culture orbital rotates about once every 24 hours and has 'gravity' about the same as Earth's, making the diameter of the ring about 3,000,000 km, and ensuring that the inhabitants experience 'night' and 'day'. Orbitals feature prominently in many Culture stories.
Planets
Though many other civilisations in the Culture books live on planets, the Culture as it currently exists has little direct connection to planet life. A small number of 'homeworlds' of the founding member species of the Culture are mentioned in passing, and a few hundred human-habitable worlds were colonised (some being terraformed) before the Culture chose to turn towards artificial habitats, preferring to keep the planets it encounters 'wild'. Since then, terraforming has become looked down on by the Culture as inelegant, ecologically problematic and possibly even immoral. Less than one percent of the population of the Culture lives on planets, and many find the very concept a bit bizarre.
This respect is not absolute though; in Consider Phlebas, some Minds suggest testing a new technology on a 'spare planet' (knowing that it could be destroyed in an antimatter explosion if unsuccessful). It should be assumed from their normal ethics, however, that this planet would have been lifeless to start with. It's also quite possible, even probable, that the suggestion was not made in complete seriousness.
Rings
Ringworld-like megastructures exist in the Culture universe but are referred to simply as 'Rings' with a capital 'R'. These habitats are not described in detail but one is recorded as having been destroyed (along with 3 Spheres) in the Idiran-Culture war. In Matter, the Morthanveld people possesses ringworldlike structures made of innumerable various-sized tubes. Those structures encircle a star just like Niven's Ringworld and are about the same size.
Rocks
These are asteroids and other non-planetary bodies hollowed out for habitation and usually spun for centripetal 'gravity'. Rocks (with the exception of those used for secretive purposes) are described as having faster-than-light space drives, and thus can be considered a special form of spaceship. Like Orbitals, they are usually administered by one or more Minds.
Rocks do not play a large part in most of the Culture stories, though their use as storage for mothballed military ships (Pittance) and habitats (Phage Rock, one of the founding communities of the Culture) are both key plot points in Excession.
Shellworlds
Shellworlds are introduced in Matter, and consist of multilayered levels of concentric spheres in four dimensions held up by innumerable titanic interior towers. Their extra dimensional characteristics render some products of Culture technology too dangerous to use and yet others ineffective. They were built millions of years ago as vast machines intended to cast a forcefield around the whole of the galaxy for unknown purposes. However, the species that developed this technology are now lost, and many of the remaining shellworlds have become inhabited, often by many different species throughout their varying levels. Many however still hold deadly secret defense mechanisms, often leading to great danger for their new inhabitants, giving them one of their other nicknames: Slaughter Worlds.
Ships
Main article: Ship types (The Culture)
See also: List of ships (The Culture)
Ships in the Culture are intelligent individuals, often of very large size, controlled by one or more Minds. The ship is considered the Mind's 'body'. Some ships (General Systems Vehicles) are tens or even hundreds of kilometers in length and may have millions or even billions of residents who live on them full time, and together with Orbitals represent the main form of habitat for the Culture. Such large ships may temporarily contain smaller ships with their own populations, and/or manufacture such ships themselves.
In Use of Weapons, the protagonist Zakalwe is allowed to acclimatise himself to the Culture by wandering for days through the habitable levels of a ship (the GSV "Size Isn't Everything" described as over 80 kilometers long) eating and sleeping at the many locations which provide food and accommodation throughout the structure, and enjoying the various forms of contact possible with the friendly and accommodating inhabitants.
Spheres
Dyson spheres also exist in the Culture universe but are only mentioned in passing and are simply called 'Spheres'. Three spheres are recorded as having been destroyed in the Idiran-Culture war.
In Matter, the Morthanveld Nestworld of Syaung-un is a 'Sphere World' consisting of a complex arrangement of transparent tubes within tubes, all revolving around a small central star. The Nestworld is alleged to contain forty trillion Morthanveld, more intelligent beings than on all the Culture and associated worlds put together. There are also noted to be other Nestworlds, but none as big as Syaung-un.