Active Galactic Nuclei?

[Dysprosium]

Can anyone inform me as to what an "active galactic nuclei" is? Thanks a lot!

[Nereid]

Simple intro:
http://imagine.gsfc.nasa.gov/docs/science/know_l1/active_galaxies.html

More detail:
http://www.ast.man.ac.uk/~rb/agn/

Google on 'AGN active galaxy' and you'll get lots of links ...