Am not a fan of campers myself although I wanted to make sure I had the right idea.
1. What's the difference of a sniper & a camper?
I Personally would think this maybe a hard one to judge.
2. Is sitting in your base on capture the flag or similar game modes and defending camping?
The statement above is what I believed to be allowed. In that, if you're in your own base on a capture the flag or protecting a bomb in domination etc. then that would not be classed as camping. If however, I was sitting in the random spot of the map with nothing to defend and I'm waiting for people to run past then that I would say is camping.