A Satellite Halo and a Planetary Halo are megastructures built in orbit around terrestrial planets or gas giants. They are called Halos here to distinguish them from natural-occurring planetary rings, and from star-centric Ringworld Megastructures.
Tech Level: 16
A Satellite Halo is a ring of loosely orbiting objects around a planet that becomes so dense with satellites that it forms a visible ring around the planet. The Halo will be thick and dense enough to cast a twilight-like shadow on the planet's surface, if in a proper orbit to intercept sunlight.
A Satellite Halo may be an unplanned development, as simple as the planet's geosynchronous orbit (for Earth, 22,300 miles above the equator) becoming choked with communications satellites and accompanying debris, slowly built up century after century.
More than likely, however, a Halo will be the result of meticulous planning with a definitive purpose in mind. Instead of a hodge-podge of large independent communications satellite, for example, a planet's civilization might instead opt for hundreds of thousands of cheap, carefully-herded microsatellites in geosynchronous orbit to accomplish the same task. In some science fiction works, Halos are created by thousands of large space colonies and attending structures in orbit, all in a tight orbital band to facilitate traffic, trade, and maintenance among them. Around gas giants and heavily-mined terrestrial objects, a Satellite Halo might be constructed to facilitate on-the-spot industrial exploitation of the encircled object, attending to multiple Space Elevators or similar high-volume interface traffic devices bringing up raw materials.
Some softer science fiction works have displayed Satellite Halos acting as a literal "defensive line" around a planet. While admittedly that can make for a dramatic visual, why the attackers simply couldn't approach the planet from another vector that avoided the heavily-armed Halo usually isn't addressed. However, it would be possible to create multiple Halos, each in a different altitude and orbit, to cover most of the entire planet from any approach.
Tech Level: 18
While a Satellite Halo is composed of many thousands of independent objects arrayed tightly in the same orbit, a Planetary Halo is one single, unified ring-like artifact completely surrounding the planet.
A Planetary Halo can accomplish many of the same tasks as outlined for a Satellite Halo, but of course offers a far greater volume for potential habitation and industrial machinery. A Planetary Halo with an orbit of 22300 miles and a one square kilometer cross section would have an internal volume of some 70,000 cubic kilometers. If spinning at the same rotational rate of the planet below (which would be very practical for it to support one or more Space Elevators to the surface) it would have a very modest gravity along its outermost surface, roughly one-twentieth of a G. Untethered planetary rings can of course be spun much faster to provide stronger pseudo-gravity.
One of the main problems with engineering a Planetary Halo (beyond its immense scale, of course) is progressional instability. The planet in its center will bind the Halo gravitationally, so it won't drift away from it, but the Halo will become unstable in the plane of its orbit. Like a bicycle wheel spinning on a carefully-balanced pole, it will eventually start to wobble and pull itself in one direction or another. Attitude thrusters would have to be placed at intervals all along its circumference to correct for any instability that might develop.
In addition to the most obvious source of power (solar) the ring could also help to power itself simply by deploying trailing tethers toward the planet's surface and generating electricity as they pass through the planet's magnetic field. If the ring is attached to the planet's surface by one or more Space Elevators, they can also serve this function.
A Planetary Halo could also fill some roles that a more dispersed Satellite Halo could not. For instance, it could function as a gigantic linear accelerator for launching spacecraft or even interstellar-bound cargoes at tremendous speeds, or they can be used as particle accelerators to plumb the depths of the quantum world on a scale no planet-bound accelerator ever could. The latter use was illustrated in the novels The Ring of Charon by Roger McBride Allen and Forever Peace by Joe Haldeman. An artificial Planetary Halo was also used by alien machines in Alastair Reynold’s novel Redemption Ark to manipulate a gas giant’s magnetic field.
Planetary Halo-like structures can also be found in novels such as The Fountains of Paradise and 3001, both by Arthur C. Clarke.