Interior design is the art and science of understanding people's behavior to create functional spaces within a building. Decoration is the furnishing of a space with fashionable or beautiful things.
In short, interior designers may decorate, but decorators do not design.
Space planning is a fundamental element of the interior design process. It starts with an in-depth analysis of how the space is to be used. The designer then draws up a plan that defines the zones of the space and the activities that will take place in those zones. The space plan will also define the circulation patterns that show how people will move through the space. The plan is finished by adding details of all the furniture, equipment and hardware placement.