A lighter typeface on a darker background, such as white text on a black background, is reversed type.
With white on black, the background prints and the text is knocked out so that it doesn't print, letting the white background (or paper color) show through. With color on color, both colors print but the background color is knocked out of - not printed - in the area occupied by the light colored type.
Also Known As: reverse type | knockout type
Examples: See Reversed Type Treatments in Desktop Publishing for examples of how to use reversed type effectively.