A scullery is a small room which adjoins a kitchen. These rooms can be used for a variety of household tasks, ranging from doing laundry to washing dishes, and in a sense a scullery is a sort of utility room. In modern homes, where a variety of equipment is used to accomplish the tasks once performed by household servants, this room may simply be a glorified laundry room, but historical sculleries were very busy, hot, unpleasant places to work.
Historically, the scullery was the domain of the scullery maid, one of the lowest ranking of the household servants. She would have reported to the kitchen maid or the cook, depending on the command structure of the servants in the house, and the messiest and most unpleasant tasks in the house would have been reserved for her. A typical scullery included a place to wash dishes, equipment for doing laundry, and cleaning supplies like mops, brooms, buckets, ashcans, and so forth.
As a general rule, this area is not used for food preparation, although in a particularly busy household, a scullery maid might use the space to perform basic tasks like plucking fowl, peeling potatoes, and so forth. More importantly, the room would be used to wash heavy duty pots, pans, and utensils used in cooking, while the fine china, silver, and other serving supplies would have been handled by higher ranking maids and footmen in the house.
Laundry facilities such as copper kettles and mangles would have been located in the scullery, turning the space into a humid steambath on laundry day. As a result, many have doors or windows which open to the outside, allowing the space to be ventilated, and providing easy access to a laundry line. The scullery could also be used to make soap, process preserved jars of food, and to perform other odd jobs.
A scullery could also be used for storage historically, with household staff stashing various cooking supplies and so forth in it. In the modern era, the storage function of this room is often its primary function, and many have shelving and cabinets for this very purpose.