What is a GameShark?
A GameShark is a gaming peripheral (i.e. a "cheat" device) that allows you to modify and manipulate certain factors and variables in a video game by inputting a series of numbers and letters in the form of a "cheat code". It would enable you to have infinite life or infinite ammunition, for example.
The GameShark has taken different forms depending on the console it is used for. For example, the PlayStation iteration took the form of a small peripheral which was inserted into one of the console's ports. The next-generation iteration for the PlayStation 2 came on a CD you had to load up before you started up any video game.
Most codes can be found on GameShark's official website, though there were plenty of other sites that give alternate codes. Moreover, the GameShark has competing peripherals, such as the Action Replay. Usually, the codes for one device are not compatible with the other.