Extruding a Face <Back
Click the Extrude button in the toolbar, then grab a flat face that is tilted slightly (not facing directly towards you).
Pulling up on the face will create a volume (if there isnt one already) and the face will move along its normal. If rounded or chamfered corners are adjacent to the face, they will move with the face.
Pushing down on the face can push the face right through the bottom of the volume; this creates a negative volume (hole). Faces on a hole have inward-pointing normals.
The Extrude along Local Normals button will extrude a curved face by moving each point in it along its local normal. Use this to create curved shells. If used on a flat face it will just do a flat extrude as above.