After taking pictures of people's faces, the faces attack the player, who must shoot them. As people walk by in the background, the game takes pictures of them, adding to the faces the player must shoot, and adding to the player's score. The background of the game is of wherever the player is pointing the camera. At the end of each stage, the player battles a boss. At the final stage, the player battles the enemies and the bosses of all the stages, without his/her life meter being refilled.
Source: Wikipedia, "Face Raiders", available under the CC-BY-SA License.