Assassination is a game mode in which to be the victor, one must only destroy his opponent's ACU. It is an option set before a skirmish game. It is arguably the most traditional of the game modes, as it has its roots in Total Annihilation, the predecessor of the Supreme Commander.
Because you only need to destroy the enemy ACU to win, some tactics will work that would not work in other modes. For example, a suicide mission with experimental units will work because the game will end immediately after you destroy the ACU. Other tactics, such as the infamous ACU snipe, are exceptionally deadly in this game mode.