Difference between glOrthof and glViewPort
The glViewport determins the portion of the window to which OpenGL is drawing to. This may be the entire window, or a subsection (think console game's "split screen" mode- a different viewport for every player).
glOrthof applies an orthographic projection to the current matrix, which is usually set to the projection matrix before this call. The projection matrix is combined with the modelview to produce a matrix that translates your OpenGL coordinates to screen coordinates.
This is equivalent to calling glOrtho with near = -1 and far = 1.
I'd recommend this page for more details on how viewing and transformation works in OpenGL.
Which should you use? Viewports and orthographic projections are different concerns, so you'll need a call for each. glOrthof and gluOrtho2D are roughly equivalent; know the difference and use one or the other.