The extrude option is used to create NURBS geometry. One setting in the tool is used to extrude a single curve in a certain specified direction, creating a surface. This extrude option in one direction uses only one curve.
Extrusion along a path has the advantage that the direction of the extrusion can be controlled. There is however a disadvantage of generating a surface like this. The profile ( section ) of the surface generated isn’t constant if the curve along which it is extruded curves to much. To generate this geometry select all the settings on the right of the option box of the command.
First select the circle you want to extrude, then select the curve. And as final step select Extrude.