Once the Extrude option is active another option called Along Facet Normal will appear. This option is ON by default. Along Facet Normal forces the extrusion to move parallel to the normal of the face/facet.
The normal is the direction to which the face is oriented. It is the imaginary line, perpendicular to the center of the face extending out into space.
When Along Facet Normal if OFF you can extrude the face by dragging it anywhere in space, by dragging the associated nodes or you can use the Delta X, Delta Y, and Delta Z fields to specify a location for the extrusion.
...