There are many ways to keep food warm at a party, including using chafing dishes, keeping an oven or roaster on low heat or using portable insulated bags to store food. These methods may be more or less effective depending on the types of food that need to be kept warm and on whether the party venue is indoors or outdoors.
Chafing dishes are large metal dishes set above a steady gas flame which keeps the food within at a relatively constant temperature. They are often used in buffet lines and make a useful tool for keeping large amounts of food warm over long periods of time. They are covered to keep food from drying out.
An oven on low heat is a good storage space in which to keep food that is best served warm. Food can be stacked inside the oven on plates or trays so that the ambient heat within will keep it from cooling down.
Insulated containers and bags can help keep food warm when it must be moved. If a cook is bringing food outside in large amounts or must move food from a kitchen to a secondary party location then insulated containers can help to prevent heat loss along the way.