It is considered that UNFAIR play consists of
-cheating
-using an aimbot (like ratbot or zbot),
-shooting players right off the respawn point,
-shooting obviously lagged (floating) players,
-shooting people that turned their faces against a wall cause they're chatting.

Something that does not belong to fair/unfair is 'CAMPING' which is highly disliked. Campers are like spiders. They sit on a place and won't move until somebody shows up right in the crosshair. Campers usually prefer places where their victims cannot escape easily (like elevators or ladders) or where it's dark so they ambush you.
Campers will be spotted soon and be shouted at, while good and skilled players raid them, as they're easy frags: After being shot by a camper, they'll return prepared and pop a grenade or launch a rocket into the camper's area and 'splatter' goes the camper. So be warned.

I know it's pretty hard to keep smiling if you are defeated all the time, BUT: keep smiling and it won't be that bad.