Canon is even worse, 5DmkII is a Full Frame camera. Then they come out with the 7D which is not "Full Frame" It should have bin a 60D or 70D.
Among the camera brands, Canon is the simplest in terms of nomenclature. Their system hasn't deviated very much since I figured it out a few years ago. n is a number from 1 to 10.
nD - "Big dawg" status. The lower the number, the better the status of the camera amongst the family. The best part of this class is that the number doesn't change, Canon just denotes the generation via the Mark number - very simple.
nnD - Enthusiast line. This is about position, not specs. That's why the 60D (even though some users claim it's a downgraded 50D) rightfully belongs here.
nnnD - Consumer (Rebel) line.