CVD's this might help or it will mess you up...(hopefully help)
The threads are where you screw on the wheel. The CVD reduces slop by making one of the dog bone ends "solid" (they are not really solid since they rotate and move and the joint) CVD's are generally stronger than the dogbone setup also. So yeah they are improved dogbones, although they will cost you more bones too...(more money)
Diff: yes they are a bunch (I cant give an exact number because there can be different numbers depending on the diff) of gears that transfer the power from 1 input to 2 outputs. Most are cased in plastic, although Aluminum cases are avaible(I wouldn't advise using one though).
We all didn't know this info at some point and had to learn it, your learning time is now and there is nothing wrong with that. Keep asking questions and you will keep learning