Extrude and Extrude Inner basics.
The Extrude and Extrude Inner tools are both usable on editable objects. They are accessed by right clicking in a viewport while in point, edge or polygon selection mode. Keyboard shortcuts can also be used, (D) for Extrude and (I) for Extrude Inner.
The extrude tool is used to create extra polygons that are 'pulled out' following the objects normals. Very useful for creating object rims, protruding shafts for items such as vehicle steering wheels.
Extude inner creates extra edges parrellel to the edges of the polygon selected as shown below:
Using this tool in combination with the Extrude tool can be very useful. If you use the extrude inner tool to create a smaller edge, you can then use the extrude tool to create a rim for an object. This is done by selecting the newly created polygons.
Then using the Extrude tool, you can pull these polygons upwards to create a lip or a rim around the object.