Whats a game "Trainer" do?
Trainers in Multplayer games=hacks(Hax) Trainers in Single player games= Cheat codes/modifiers
trainers are for modifying the game. not necessarily hacks. watchout though, they can be detected and often times get banned from online games using them. look at counterstriek source :P
they also make you lag. so dont use them multiplayer wise.