This How Stuff Works article is an informational article all about how to cheat in video games. It covers many different aspects of cheating such as using codes, hacks, outside hardware, as well as cheating in online games using techniques like auto-aim or macros. The article points out that sometimes poor game design prevents the user from completing certain tasks within a game, so the only option is to cheat if you want to complete the game.
Another point that the article makes is that cheating is not always used to make the game easier. Some people use cheat codes or alter the code of the game itself in order to make the game more challenging by making the enemies have more health or better accuracy for example.
Finally, the article makes the distinction between cheating in single player games versus cheating in multiplayer online games. When you cheat in a single player game, you are only changing the game experience for yourself so it is essentially harmless. However, when you use macros or hacks in online games you not only change the game experience for yourself, you may also be infringing on the enjoyment of the other players in the game.
http://electronics.howstuffworks.com/how-to-tech/how-to-cheat-at-video-games.htm