For orthographic parallel projection:   

glOrtho(left, right, bottom, top, near, far); 

glOrtho2D(left, right, bottom, top);   

Here left, right define the x-direction extents, bottom, top define y-direction extents and near, far define the z-direction extents of the view volume. gluOrtho2D is exactly like glOrtho, but with near = -1 and far = 1. That is, gluOrtho2D views points that have z-value between -1 and 1. Usually, gluOrtho2D is used when drawing two-dimensional figures that lie in the xy-plane, with z = 0.