This here gives a rough outline:
http://www.nhtsa.dot.gov/people/injury/enforce/cvm/CMV_license.html
A listing with most of the state's individual cdl requirements.
http://www.dmv.org/cdl-education.php
States vary so widely, in CA if you're the pizza delivery
boy, you're supposed to have a class D cdl. If you have a
monster RV, you need a class B or class A noncommercial
endorsement.
As far as vehicle determination for licensing and bridge
weights, the door tag GVW seems to be the standard that
the enforcement folks go by, that and registered weight.
Farm? agriculture? tree farm/nursery, uhh, that's a whole
nother pile of exemptions and requirements.
I was suprised with my first dually, had to have comm plates,
fire extinguisher, triangles and wheel chocks.. sheesh.
-Jason