Cool. I assume that the numbers increase with upgrades? So a D80 is newer than a D70 which is newer than a D60?
Sort of, I had a D50, Nikon then brought out a D40 and then a D60 so they sort of screwed things up slightly there. You've also got the D300 (due an upgrade soon so probably D400 this year maybe) and D600, the D300 is a semi-pro crop only body the D600 is a semi-pro full frame body so who they're actually aimed at I dunno. Oh yeah then there's the D800 which well is a high res semi-pro full frame body yet has a high resolution than the top of the line pro model the D4. Confused yet? It is quite a weird system, but I think they're trying to more categorise the cameras by what they're meant for, the D4 is an all round pro body, the D800 more for the videographer and well everyone else too.
Have a look at these wiki pages, shows the current cameras and what market they're aimed at quite well:
http://en.wikipedia.org/wiki/Comparison_of_Nikon_DSLR_camerashttp://en.wikipedia.org/wiki/Nikon#Digital_single_lens_reflex_cameras