I think the distinction is normal, named, elite, boss...

in order of difficulty...been a while, but I think that's what is was when I last played. So, a mob can be named but not elite, elite is more like paragon in UO and boss is more like peerless.

The rare is just as easy as the normal version of it's level...but you're usually guaranteed a green drop or something nicer from it. The elites are much more difficult for their level...loot is not special, but they give extra XP.