Accretion is a phenomena that plays a role in variety of astrophysical objects. The first type of object I was interrested on are Tidal Disruption Events, or TDE for short. TDE are violent phenomena that arise when a star wanders too close to a super-massive black hole. When the tidal forces exerted by the black hole become larger than the self-gravity of the star, the star is disrupted. Some of its matter will be gravitationnally bound to the black hole and form an acretion disc.
In these discs, matter will slowly fall towards the black hole while the angular momentum of the gas will be transported outwards. In doing so, the gas will be grealty heated by its viscosity and will thus emit light (the simplest model is to assume this emission to be black-body emission). More refined model can also take into account specific line emission of molecules whithin the hot gas, and even synchrotron emission from electron in the plasma.
However accretion is a phenomena arising in more systems than just TDE formed discs. Larger, brighter accretion discs in the centre of galaxies, around their supermassive black hole are also refered to as Active Galactic Nuclei (AGN). Even though the scales of those discs are larger than those formed by TDE, the physics at hand is subtentially the same. However more violent phenomena may also occur in those larger disc, for instance ultra relativistic jets. In a less violent fashion, but not simpler by any mean, protoplanetary discs (like what happened in our solar system), also are accretion dominated discs. Discs in binary systems like cataclysmics variable stars (or CV for short), where a white dwarf progressively absorbs its companion star, are also accretion discs. The main difference between these types of accretion discs is the mass of the object in their centre. Subject to different gravitationnal potential, the discs will reach more or less dense state, thus higher or lower temperature, on which their emission direclty depends.