The viewport is the area of the window that displays an image. You specify it in window coordinates, where the coordinates of the lower left corner of the window are (0, 0). A more technically accurate definition of the viewport is that it is the mapping from normalized device coordinates (NDC), the coordinate frame in which the 3-D clipping is performed, to window coordinates.
Because 3-D clipping occurs in NDC, no drawing primitive is ever drawn outside the boundaries of the unit cube in NDC. Thus, when the unit cube of the NDC is transformed to window coordinates, no drawing primitive draws outside the viewport. An exception to this are text strings. Refer to "Creating Text Characters" for more information.
For more information on using viewports and screenmasks in GL, see: