GDDM, Graphical Data Display Manager, is a graphical, character-encoding
scheme used for displaying graphics. Two other graphic display schemes are
Vector and All Points Addressable (also known as "APA" or "Raster").
Using GDDM to display a box on a display screen might consist of sending
four character codes. You might expect the four codes to consist of two
parallel-horizontal lines and two vertical-parallel lines, all with end
points touching. But instead, one of the character codes might look
something like the letter "L". The second would be an "L", but flipped
upside down. The third would look like an "L" flipped horizontally, and the
fourth character would display like an "L" flipped vertically and
horizontally.
Therefore, these "L"s could be used to construct a rectangle on the
display. GDDM uses a variety of graphical characters, some looking like
"L"s, to construct graphical displays.
Vector graphics, of course, define lines bound by two points. APA (Raster)
graphics provide control of every point.