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Active galaxies are distinguished from other galaxies in that they show indications of having energy output not related to ordinary stellar processes. The activity is centered in a small nuclear region and associated with strong emission lines. The nuclei of such galaxies are named Active Galactic Nuclei (hereafter AGNs). This group of objects includes bright quasars, with luminosity exceeding 1047 erg s-1, as well as faint LINERs emitting no more than 1041 erg s-1. For the purpose of this work an object will be classified as an AGN if at least one of the following is observed:

a: Compact nuclear region, brighter than the corresponding region in galaxies of similar Hubble type.
b: Nonstellar nuclear continuum emission.
c: Nuclear emission lines indicating excitation by a nonstellar continuum.
d: Variable continuum and/or emission lines. AGNs are further classified into subgroups, according to their luminosity and spectrum. The following main groups are usually identified: