The spectra of Seyfert galaxies are characterized by emission lines from highly ionized gas. Within the nuclei of Seyfert galaxies, then, there exists very hot gas which is swirling around very rapidly. The luminosity of the nucleus of a Seyfert galaxy can vary wildly on timescales of only a month. Thus, a very large amount of energy is emerging from a very small volume. Aside from their bright nuclei, Seyfert galaxies look like ordinary spiral galaxies.
Some active galaxies eject long jets of gas from their nuclei.
As radio galaxies go, M87 is very weak. The jet in M87, in addition, is smaller than the galaxy itself. In a photograph taken at visible wavelengths, Cygnus A looks like a distorted elliptical galaxy, a few kiloparsecs across. At radio wavelengths, however, we see a pair of tremendously long jets of gas, which dwarf the central galaxy of stars. The energy source for an active galaxy is a central supermassive black hole.