Accretion disks are a phenomena found primarily in binary systems. Accretion disks get their names from the meaning of "accretion" which is the act of accumulating. Accretion disks are formed when one of the stars in a binary system develop into a black hole or a neutron star. The intense x-rays created by the accretion disks are one of the only means of finding a black hole or neutron star in distant galaxies.
The accretion disk is actually formed when the gravity of a star or black hole in the binary system dominates the other star's roche lobe. Then the matter obtained from the less dominant star accumulates around the dominant black hole or neutron star, forming the accretion disk.