There are different ways to cheat in games;
Bug exploiting is often (sadly) common in MMORPGs because it is very hard to make a bugfree game, especially a MMORPG where the players have the freedom to do a lot of things in the game. Another type of cheat is to use external applications which allow you to have different kinds of bots that performs different tasks for you in game. Use of external programs is against the Terms of Service (ToS) and if you’re caught using one of these programs your World of Warcraft account will be banned.