All the elements you add to your Revit model are organized into families. This includes walls, doors, structural members, mechanical equipment, or annotation elements such as elevation symbols, door tags, and column gridlines.

A family is a collection of elements with identical use, common parameters, and similar geometry. For example, although you may have different sizes of desks, all the sizes can belong to a desk family. If you look in the Project Browser, you can see a branch called Families.