A bug, also known as a glitch or a fault, is an unexpected error in the code of a computer program. In video games, these are almost always negative and can range from being inconvenient, to making the game unplayable. Occasionally, a bug may be beneficial to the player, in which case it is often labelled as an exploit, and using it is often considered a form of cheating. In MMOs, such exploits often result in a ban if you are identified as having used it.
Some examples of detrimental bugs include ones that can corrupt save files, and those that leave the player in a position where it is impossible to continue the game. Beneficial bugs include exploits such as those that allow a player to quickly gain large amounts of that particular game's currency, or explore areas that would normally be inaccessible (though in some cases, the latter may also prevent the player from returning to the proper areas, forcing a reset of the game).
In some cases, what might seem like a bug may simply be considered to be an undocumented feature, though it can generally be considered a bug if it causes the game to behave in a way that is contrary to what the player would expect from a normal gaming experience.