Usually, both rectangles are aligned with the coordinate axes.
Window has corners and ;
Viewport has corners and .
Length and height of the window are and ,
Length and heigth of the viewport are and .
and similarily for .
Readings: Hearn and Baker: Section 6-3. Red book: 5.5. White book: 5.4, Blinn: 16.
Intuitively, the window-to-viewport formula can be read as: