They are Rings but smaller. Culture Rings are like Niven's Ringworld, and go all the way around a star, spinning around the star to simulate gravity. Culture orbitals are also ring-shaped and spinning to simulate gravity, but they're nowhere near as big (relatively speaking; still more surface area than Earth), and generally orbit a star more like a planet does. Culture Spheres are Dyson spheres.
The size of an Orbital is dictated by day length and surface "gravity" - they are arguably more elegant than Niven style Rings which need a lot of extra stuff to generate day/night cycles (shadow squares?).
I don't think the Culture builds rings or spheres themselves, but they exist and they are aware of them. There are civilizations in the galaxy considerably more advanced than the Culture.
