An active galactic nucleus (AGN) is a compact region at the center of a galaxy that has a much-higher-than-normal luminosity over at least some portion of the electromagnetic spectrum with characteristics indicating that the luminosity is not produced by stars. Such excess non-stellar emission has been observed in the radio, microwave, infrared, optical, ultra-violet, X-ray and gamma ray wavebands. A galaxy hosting an AGN is called an “active galaxy“. The non-stellar radiation from an AGN is theorized to result from the accretion of matter by a supermassive black hole at the center of its host galaxy.

Speciality

Active galactic nuclei are the most luminous persistent sources of electromagnetic radiation in the universe, and as such can be used as a means of discovering distant objects; their evolution as a function of cosmic time also puts constraints on models of the cosmos. Many AGN lie at very large distances from us, at high redshift. In particular, the existence of very distant Seyfert galaxies giving off gamma-ray glows indicate such objects exist everywhere in the universe. 

Quasars!

 Quasars (short for “quasi-stellar radio sources”) are the most energetic and distant active galactic nuclei known. Astronomer Carl Seyfert (1911–1960) first wrote about these so-called “active galaxies” in 1943. Their strong emissions indicated something very energetic was going on the central cores. Eventually they became known as Seyfert galaxies.

Types of Active Galaxies

Active galaxies are characterized by the emissions they give off and whether or not they emit jets from their cores. Here are a few of the most common types.

Radio-quiet: very dim, quiet galaxy cores with radio quiet (for now) black holes; they may be bright and active in other wavelengths of light

Seyfert galaxies: medium-mass black holes accreting material and giving off x-rays and gamma rays 

Quasars: high-mass black holes accreting material; some emit radio emissions while others emit only optical light Blazars: high-mass black holes with a jet pointing toward Earth 

Radio galaxies: high-mass black holes with large areas that give off strong radio emissions and have massive jets streaming superheated material into space.These powerful jets appear to be moving faster than the speed of light—a property called “superluminal motion.”

Uses of Active Galaxies

X-ray emission from active galactic nuclei have given astronomers many clues about what is going on in these galaxies. Early X-ray observations of AGN showed fairly simple sources that could change brightness over fairly short timescales. Such variability pointed to emission coming from a fairly small area. The rapid changes, high energy output, and small volume all pointed to a black hole accretion powering these galaxies – it is one of the only things that can put out the amount of energy we see from AGN in such a small volume.

Since X-rays originate from very close to the central black hole, X-ray studies give us a unique view of the processes at work in the very center of the action. In some cases, higher energy X-rays have the ability to punch through gas and dust, so this is one part of the electromagnetic spectrum that lets us see into highly obscured AGN.

Like any other massive object, black holes can pull in matter that ventures too close. If there is enough infalling matter, it can form an accretion disk. This disk of matter surrounds the black hole and heats up, emitting X-rays. As matter makes its final plunge into the black hole, it is accelerated to high velocity, causing X-ray emission. Some of the infalling matter can also be funneled away from the black hole in powerful jets along the rotation axis of the disk. These jets are observed across the entire electromagnetic spectrum.

http://en.wikipedia.org/wiki/Active_galactic_nucleus
https://imagine.gsfc.nasa.gov/science/objects/active_galaxies2.html