To extrude a face, hold the Alt key down, left click on the face and drag the resulting prism away from the original face.  To extrude a face in the perpendicular (normal) direction, Alt-left click on the face while holding the mouse still, then use the Multiply (*) and Divide (/) keys to extrude the prism.  All extruded faces assume the same décor and component names as the original faces.

Extruding a group of faces, or all faces in an object, is done in the same way as cloning, except that the Alt key is used instead of the Ctrl key.

An alternative way of extruding faces is by ‘drawing’ them out with the ‘Pliers Cursor’, as described under Pliers Mode.

You can also move extruded faces with the Esc and Tab keys.  In this case, the face vertices all move along their own radials, thus enlarging faces with the Esc key as they move away from the origin, or shrinking them with the Tab key as they move towards the origin.  If all faces are selected, the entire object enlarges or reduces.  Experiment a bit to see some remarkable effects.