Active galactic nuclei are non-stellar sources of radiation located at the centers of galaxies, and quasars are the brightest examples of such activity. In quasars the nonstellar emission can be up to thousands times brigher than emission from the stars of the host galaxy. This emission covers a very broad range of the electromagnetic spectrum: from radio and infrared emission, through optical and ultraviolet, up to X-ray and gamma-ray band.

This non-stellar emission comes from the matter falling into the supermassive black hole located at the dynamical center of the host galaxy. The mass of the black hole ranges from a milion to several bilions of the solar mass. The level of activity depends on the amount of the available material which has to loose the angular momentum in order to accrete to a central black hole. The inflowing material heats up and radiates intensly, the inflow is accompanied by some outflow in the form of a wind or well collomated spectacular jet which can extend to distances many times larger than the size of the whole galaxy. Jet emission is well seen in the radio maps, but the active nucleus itself is unresolved. We can study it by measuring the object spectra, including polarization, and using source variability. Fortunately, all active galaxies are strongly variable.

Quasar emission consists not only from the continuum emission, but also of numerous spectral features. In particular, broad emission lines seen in the optical and in the ultraviolet allow to determine the quasar redshift and to measure the black hole mass.