The AGN information, derived from Sloan data, were published by Dr. Guinevere Kauffmann and her group at the Max Planck Institute for Astrophysics.
Prof. Robert Brunner, Nick Ball, and Brian Wilhite of the UIUC Astronomy Department gave advice on retrieving, using and interpreting this SDSS data and Kauffmann's derivatives of it.
At first we see galaxies only (333000 of them) -- shown in yellow/orange/reddish.
Turning on "AGNs" (click on the labelled button) puts ring markers on galaxies (about 70000 of them, nearly a quarter of all those surveyed) with detectable active nuclei.
Ring color shows relative brightness of AGN to the host galaxy: blue ones' nuclei outshine their host galaxies by a large margin (at least 5x), brown ones are fainter (0.5x or less). Ring size (actually area) is proportional to AGN core luminosity. A few large rings mark especially bright nuclei; most are relatively dim, and aren't visible unless you fly close to them. Switching on "agnhostgals" puts a dot at every galaxy with an active nucleus, even faint ones.
Turning on "Quasars" puts crosses on objects the SDSS survey classifies as such. These are mostly extremely luminous and very distant, much farther away than most of the galaxies in this sample.
Their size is again related to luminosity of their active nucleus, though the scale factor is smaller to keep the very bright quasars from completely swamping the view. In their case, color is also related to luminosity: the bluest quasars are the most intrinsically bright.