Single player games it's only cheating if it feels that way to you. If you have fun putting in cheat codes or even using Gameshark, it's all good.
In multiplayer, anything that gives one side an unfair advantage is cheating. Cheat codes for one side, infinite combos that can only be performed on one side of the screen, stuff like that. If the same tactics or abilities are available to both sides, it isn't cheating. At that point, it's just a preference. Does everyone playing want to have one hit kills turned on, or no? Stuff like that.