Simple Definitions :
NET AREA is the area required for specific uses such as; classrooms, laboratories, offices, conference rooms, auditoriums, dining rooms, storage rooms, shops and other functional spaces. Net area is measured to the interior face of enclosing walls.
CIRCULATION AREA is the area required to support the function located in assignable spaces such as; lobby, corridor area, toilet area, utility spaces, staircases, mechanical & electrical rooms, lift, risers & other service areas.
GROSS AREA is the total area of the building measured to the exterior face of exterior walls.
Simplest Definitions :
GROSS = NET + CIRCULATION