These actions are usually considered to be easy, crafty ways to get a desired result and may involve breaking written and social rules or involving things that aren't even relevant to the issue at hand if convenient to quickly make things difficult for the opponent and 'take heat off themselves'. They may also be used when the user simply has no legitimate or fair moves, points or reasons to get their way and desired victory or glory or might not even deserve it, but to someone willing to use a cheap shot "a win is a win, even if everyone else thinks I'm a scumbag. They're just jealous!"
Ad hominem is considered a cheap shot in debate
Media using images or statements and taking them out of context for profit or attention or making libel can be considered use of cheap shots
Scratching, biting, hair-pulling, kicking males in the testicles and many "street fighting" techniques can be considered fighting dirty
Firing on unarmed people
Yeah, that was a cheap-shot...
I waited for him to turn his back on me, then nailed him in the back of the head.
"Cheap-shot!" Everyone yelled.