When an entity (the shooter) shoots, it performs multiple actions. The primary action is that the shooter releases a projectile object. The projectile object moves along a trajectory. This trajectory is often determined by the shooters targeting. However, in some games such as Artillery Duel (and more recently, Worms), entities shoot by determining an angle to shoot along, and a power level that determines the force behind the projectile's movement. Also, entities that are not agents often shoot without any sort of targeting.
The purpose of shooting is usually to make the projectile collide with some other entity. The result of this collision varies from game to game. The collision could remove the shot entity. The shooter could capture the entity that collides with the projectile. A collision could decrease the value of an attribute of the shot entity, such as decrease its health attribute (ie, many first-person shooters).
In Galaga, the player-controlled entity can shoot bullets. The player-controlled entity cannot shoot another bullet if the last bullet it shot is still on the screen. If the bullet collides with a non-player entity, the bullet and the non-player entity are removed. The player-controlled entity can aim its shot by moving horizontally.
In Artillery Duel, the player-controlled entity can adjust it's angle and power in order to aim its shot. Once set, the player-controlled entity can shoot a bullet. The trajectory of the bullet is determined by the angle and the power as well as the wind speed and direction. If the bullet collides with another player-controlled entity, the shooting entity scores a point.
In Halo, Master Chief must take gravity and distance into consideration when throwing a granade. Although it's different than shooting, the player still launches a projectile. When the granade reaches its destination it explodes.
In Street Fighter a player may invoke a special punch that acts like a projectile of energy. This is a weak example for several reasons: 1, there is no targeting, except for facing the opponents direction, 2, there is no physical projectile; in the game it is considered a special punch, 3, the special punch does not kill or capture the opponent, it only lowers its health.
In Kirby's Adventure, Kirby is able to gain various types of projectile abilities by eating certain types of enemies (Cutter for example). However there is no way to target an enemy or aim your projectiles at an enemy as you travel along the 2D level. Kirby is only able to shoot directly in front of him while he is on the ground.
Super Mario Brothers
In any of the Mario Brothers games when Mario shoots using the fire flower. Although there is no real aim and the trajectory remains the same no matter what movement Mario makes, if one of the fireballs touches and enemy, the enemy is removed from the game. This is considered a weak example because of the limitations the player has on aiming and trajectory of the fired projectile.