When examined spectroscopically, galaxies are generally found to have absorption features similar to those found in stars. This is what one would expect for the integrated light of a population of normal stars. However, in 1943 Carl Seyfert identified a class of spiral galaxies (later given the name ``Seyfert'' galaxies) which exhibit broad forbidden emission lines of [O II], [O III], [N II], [Ne III], [S II], and [S III], coming from unusually bright central regions. The nuclear regions are so luminous that they account for much of the galaxy's light, often having the appearance of stars in photographic plates because they so strongly outshine their spiral host galaxies [Seyfert, 1943].
Seyfert galaxies are characterized by their incredibly bright nuclear regions and their prominent emission lines. The widths of the forbidden lines are typically >~1000 km/s. The permitted lines of hydrogen and helium sometimes have the same width, and sometimes are much broader with widths corresponding to a few percent of the speed of light. Seyferts that have such broad lines are classified as type 1, while those without are defined as type 2. Type 1 Seyferts are also much stronger keV X-ray emitters, and tend to exhibit stronger and more rapid variability in this waveband [Mushotzky et al., 1993]. In some Seyfert galaxies, the permitted lines exhibit both broad and narrow components. These intermediate Seyferts are classified as types 1.5, 1.8 and 1.9, based upon the relative strengths of their broad and narrow H I components [Osterbrock, 1989, pages 312-313,].
Seyfert galaxies fall under the broader category of active galaxies. The defining characteristic of active galaxies is the presence of an extremely energetic process in their nuclei. The power of these active galactic nuclei (AGNs) is so extreme that a region less than a parsec across releases more energy than the entire rest of the galaxy, which may span tens of kiloparsecs. AGNs can outshine entire galaxies not just in visible light, but across the entire electromagnetic spectrum. Their X-ray output is indicative of temperatures orders of magnitude hotter than the surfaces of stars. Their radio emissions can be so powerful that the brightest objects in the radio sky are active galaxies, despite the fact that they are also some of the farthest objects known.