In Film

Set dressers arrange objects on a film set before shooting. They work under the direction of a leadman

Leadman

A leadman is a Set Decoration department member who is responsible for the props and swing gang and/or the set dressers on a film set. The swing gang does the set dressing and wrapping. Set dressers keep the set in the proper condition by placing and moving elements and props as needed for the...

, a set decorator

Set decorator

A set decorator is in charge of the set dressing on a film set, which includes the furnishings, wallpaper, lighting fixtures, and many of the other objects that will be seen in the film. Props and set dressing often overlap, but are provided by different departments...

 and a production designer

Production designer

In film and television, a production designer is the person responsible for the overall look of a filmed event such as films, TV programs, music videos or adverts. Production designers have one of the key creative roles in the creation of motion pictures and television. Working directly with the...

. Set dressers place furniture, hang pictures, and put out decorative items. They are also responsible for some light construction and assembly of small items, such as air-conditioning ducts. They also move items as necessary to make room for the filming equipment. During the shoot, the prop department works with an on-set dresser to ensure that the prop

Theatrical property

A theatrical property, commonly referred to as a prop, is an object used on stage by actors to further the plot or story line of a theatrical production. Smaller props are referred to as "hand props". Larger props may also be set decoration, such as a chair or table. The difference between a set...

s and furnishing are in the proper location for the script and to maintain continuity

Continuity (fiction)

In fiction, continuity is consistency of the characteristics of persons, plot, objects, places and events seen by the reader or viewer over some period of time...

, as scenes are often shot out of order.

In Theater

Set dressers "dress" the set of a play. Set dressers specifically deal with items on stage that are not interacted with by actors. Often set dressers are in charge of finding and installing curtains, wallpaper, photos, bedspreads, rugs and other items onto the set.
Set dressers are sometimes a part of the props department, construction

Set construction

Set construction is the process by which a set designer works in collaboration with the director of a production to create the set for a theatrical, film or television production...

 department or their own department. Whether set dressing is its own department is often dependent on how much money a theater has. Set dressers also often have a lot of cross over work with the props department or construction

Set construction

Set construction is the process by which a set designer works in collaboration with the director of a production to create the set for a theatrical, film or television production...

 department.
The set dressing department works under the director, producer and scenic designer.