Seyfert galaxies differ from other active galaxies (most notably quasars) in that their galactic nuclei are lower in luminosity compared to the rest of the galaxy. Quasars have nuclei that easily outshine the rest of the galaxy. Seyfert galaxies, on the other hand, host active nuclei that do not outshine the rest of the galaxy by the same amount.
Interestingly enough, in his original analysis, Seyfert focused on emission lines, noting that in this class of galaxies, there are strong high-ionization emission lines present in certain parts of the spectrum. This alternate definition is also used today. LINERS (Low Ionization Nuclear Emission line RegionS) are very similar to Seyfert galaxies if this definition is used; they are differentiated because LINERS also contain low-ionization emission lines.
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