Well, the D3xxx (D3000, D3100, D3200) series is pretty much entry level, easy to use, guide mode supported. The D5xxx (D5000, D5100, D5200, D5300) series I guess you could say is more of an enthusiasts type camera, more options that you can control without the menus but still easy enough for entry level. The D7xxxx (D7000, D7100) is pretty much top of the line for their crop sensor cameras, a lot of manual controls available without using the menu, more options for manual shooting than the 5xxxx series, etc.
If it has 4 digits in the number, 3000, 5100, 7100, etc then normally it's considered more of a "consumer" or "prosumer" grade camera - although the line is getting blurrier and blurrier here as time goes by, for example the D7100 and D7000 both have some of the higher quality build features like weather sealing, etc.
If it has only 3 digits then generally speaking it's usually a more professional grade camera, with less automatic and more manual support, better build quality, faster shooting speeds, etc. Their full frame offerings like the D600, D610, D700, D800, D800e - are all pretty much in this category. You'll also see some older non full frame offerings like the D300, D200, etc that also feature this higher build quality but are not full frame.
Their absolute, top of the line flagship stuff is generally a single digit, D3, D4.