The design area is the white area inside the software's screen. It has a border that serves as a guide and represents the size of your substrate. The size of the design area does not limit the size of your design or where the design is placed on your media during output.
Margins can be placed inside the design area. Those margins are used to distribute and align objects inside the design area. You can change the size and the color of the drawing area and show or hide the borders. (See DesignCentral - Document Tab for more information on how to set up your document properties.)