The accretion disk of a black hole is a kind of ring around the black hole, and is mostly made of gas.

Accretion disks often form in binary systems, where one of the bodies is a black hole. Its gravity pulls matter from the other star, and some matter falls in directly, while the rest miss a little and start to orbit the black hole. The gas accumulates over time. The friction between the matter in the accretion disk causes it to heat up, and release X-rays or even gamma rays. These are known as X-ray binaries, or XRBs.

The other place accretion disks usually form are centers of galaxies, around supermassive black holes. These black holes have much bigger accretion disks that make it release more light and heat, making it often brighter than most stars. These are known as quasars. Sometimes, they release jets of matter outwards near the speed of light, and these jets always release in the axis of rotation.