What is the difference between a serial key and a cheat code

"Serial key" usually refers to a unique code that is designed to activate purchased software. Many programs are available as shareware; that is, you can freely download them but their features are limited until the software is purchased. The serial key "proves" to the program that it has been properly bought.

"Cheat code" refers to a hidden feature in software (usually games). They are intended features so they are typically not illegal to use. The word "cheat" implies that you're going beyond the storyline and/or gameplay, but the game is designed to allow such features. They're a type of 'Easter egg' (deliberately hidden features in software).

Since a serial key is purchased, providing tips on how to bypass such a mechanism is against Answers.com policy. In most circumstances, bypassing software protection mechanisms (and even the simple possession of anything that is capable of cracking protection mechanisms) is illegal. The programmers of the software do not intend for the protection mechanism to be bypassed (and in fact, they usually take measures to prevent such a thing).

However, a cheat code is built-in to the game by the programmers. They are part of the price tag on a video game. So, not only is sharing and using cheat codes on games legal, but it is even encouraged so that you can get the most out of your new game.