The 1st AD on American productions runs the set on behalf of the Director announcing the takes. When everything is ready for a take (the camera in place, the lights properly set, the actors make-ready, etc.), the 1st AD gives the command to start Sound and Camera ("Sound! Camera!"). Then the director asked the actors to play. The Director then controls the "Cutting" of a take and when it is time to move on to the next scene.

See also - Link:

http://www.vierundzwanzig.de/24_filmschule/kamera_ab/(page)/3