Location Sound Overview   

The term location sound describes the activity of recording ambiance and dialogue on location (vs. a "sound stage" or  another isolated sound environment like a studio).  For most film/video work, the first order of business is capturing the dialogue or speech: this must be to the highest standard available and it also must be in perfect synchronization to the simultaneously captured image.  A location sound recordist must also record the ambiance of the location - used to fill the gaps in edited dialogue and to provide a continuous soundscape for the scene.
On a large production the Sound Department personnel may include a   mixer ( head of the Sound Dept.) who controls levels at the recorder and chooses mics and their placement, etc., 
a boom operator - who holds the boom and positions the microphone and another 
assistant - who might pull cables, etc.