Question: In [Iain M. Banks'](https://en.wikipedia.org/wiki/Iain_Banks), fairly rockin' [Culture novel](https://en.wikipedia.org/wiki/The_Culture), [*Consider Phlebas*](https://en.wikipedia.org/wiki/Consider_Phlebas) a good third of the book takes place on what is called the **"Vavatch Orbital"**. An orbital is apparently a massive artificially created megastructure, similar to the [Death Star](https://en.wikipedia.org/wiki/Death_Star) or the titular [Halo](https://en.wikipedia.org/wiki/Halo_%28megastructure%29).

At one point in the book the Vavatch Orbital is described as a minor orbital, not as big as a ring or a sphere. Then later it is almost directly described as having a ring shape. Still earlier in the story it is described as a bucket being swung in circles, though this description maybe used only to describe how the orbital uses centrifugal force instead of actual gravity.

Whatever the size, the orbital has to have enough sea space for a Megaship (a city sized ocean ship) to travel for more than a century before reaching land again.

[Wikipedia tends to suggest that all orbitals are rings](https://en.wikipedia.org/wiki/Orbital_%28The_Culture%29), but then why the mention of a sphere, or that Vavatch is not big enough to be a ring?

Answer: 
I think it is clear from the *Culture* novels that there are various mega-structures that are found in the culture universe, including spheres, rings, orbitals, shellworlds and mega-ships like GSVs that can accommodate billions of people themselves.

It seems clear from their regular appearance in the novels, that orbitals are relatively common. @DanielRoseman's excellent answer quoting Banks describes that pretty well - but from this description and the novels we find that Orbitals are ring shaped structures, in orbit around a star (not surrounding a star). They are sometimes complete and continuous (like Vavatch) and sometimes not complete or finished, and comprise 'plates' that people live on - such as the Chiark Orbital from *Player Of Games*. Orbitals tend to have a 'hub' comprising a mind located at the point that it revolves around.

I think the description in *Consider Phlebas* is a little confusing, but it is meant to distinguish orbitals (these *relatively* small structures in orbit around a star) from a 'true' ring in the sense of a [Niven Ringworld](https://en.wikipedia.org/wiki/Ringworld), or a sphere, by which I presume is referring to a [Dyson Sphere](https://en.wikipedia.org/wiki/Dyson_sphere). A Niven Ringworld is a vastly bigger structure than an orbital - a ring at Earth orbital distance around a star would be over 900 million kilometers in circumference - much larger than the 3 million kilometers that Banks describes an orbital. Rings if they are rotated for gravity, unfortunately take the laws of physics to breaking point - Niven had to invent magical new materials for it to be made from. In the Culture however, perhaps (equally magical) force fields can be used in place of unrealistically strong materials.

I think we are meant to presume that rings and spheres are known in the Culture Universe, but perhaps much, much less common than orbitals.
As others have mentioned, it's a large ring which rotates to generate centrifugal force to simulate gravity on the inner wall. The ring is in orbit around a star like a planet orbits a star (with the star far outside the circumference of the ring itself). In this way, the inhabitants inside the ring can watch the sun rise and set as the ring turns (imagine "midnight" as the time when they are closest to the sun, their feet pointed toward the sun, and it is obscured by the floor. And "noon" as when they are furthest from the sun, their feet pointed away from it, and it can be seen roughly overhead).

Other responses have neglected to mention that the surface of the orbital is also sloped toward the edges so that the inner surface looks like a parenthesis in cross-section. (If you were on the inner surface, with both edges equal distances, you would need to "climb" to escape the ring). This is explained as the principle reason the atmosphere stays inside the ring, as there's no solid ceiling. There is also a layer of artificial dust suspended in a magnetic field to shield the inner wall from solar radiation and create different climates.