This game is simple...
[1] Definitions will appear at the bottom of the screen. [2] Use your mouse to 'shoot' the word which it describes. [3] Your score reflects the time left on the clock at the end of the round.
However...
[1] You only have 20 rounds of ammo and 30 seconds to complete each level of the game! [2] The game is over when you are beaten by the clock or when you run out of ammunition.