As Doc said.

Pain, the dog whimpers, lays down, tries to run away, tries to bite you. You will know when the dog is in "pain".

Discomfort, something that is more annoying and pests than actually hurting. Like a ear pinch, it bothers the dog, but really doesnt "hurt" them. They figure out that when they perform a certain act, the annoyance goes away, and only when they do a certain act.

Its like when your wife or mother is nagging you to take out the trash, you do it so that they will shut up, not because they are physically pulling on your ear all the way out to the can. (usually)