Second unit

Definition:

A secondary production crew will be used to shoot background footage, cut-away and cut-in shots that do not require the attendance of the main actors or director.

Uses:

Second unit crews are used to reduce costs. The shots they film are detail shots and can be completed at a time other than the main shoot. Because expensive actors and large scale production crews, such as lighting, sound, makeup, wardrobe etc, are not needed second unit shots can save money.  Many camera operators and directors gain valuable early experience in a second unit crew.