What is a video game bug

If it is a bug as I believe you mean it, then it is a glitch. An error, or something that either messes up players or something that can be exploited. These errors are usually refered to as "bugs" when they are patched (which is a fix). An example of a bug would be if a player gets stuck on a wall at a particular part in the game. Another is example is the elevator glitch in Call of Duty Modern warfare 2, which is exploited by the players to get "out of the map" and therfore gain an advantage over other players.