Cheating

Cheating happens when someone breaks rules or lies.
Exams.
Cheating can be done in many ways. Some students cheat on a test by copying someone else's answers and writing secret notes. This can get them kicked out of school.
Sports.
People who break the rules of a game are cheating. For example, if someone holds the ball when playing soccer, they are cheating. Another form of cheating in sport is using performance-enhancing drugs; Ben Johnson and Lance Armstrong are examples of sportspeople who used drugs to cheat.
Relationships.
When someone is in a relationship with another person and they kiss, date or have sex with someone else, they are cheating on the person they are in a relationship with - unless they are in an open relationship. This is called infidelity. If the two people are married, and one has sex with somebody else, it is called adultery.
Laws.
Local and national governments have laws. If someone breaks the law, they can be punished. If someone cheats in a way that is not illegal, they cannot be punished by law. Some common punishments include jail and fines.
Regulations.
Organizations, such as schools, can have rules. Rules are not laws, but people can still be punished for breaking a rule.
In games.
The first cheat in video games was thought to be the "Konami Code", a code Konami used to test a game that was too hard for them to do alone. See cheat code for more on cheating in video games. 
Sometimes, even if the game creators do not provide cheat codes, it is possible to cheating with some 3rd party software. Game cheats create unfairness in online games and often cheating players' accounts are banned.