From BlenderWiki

Extrude

One tool of paramount importance for working with meshes is the Extrude command (E). It allows you to create parallelepiped from rectangles and cylinders from circles, as well as easily create such things as tree limbs.

Although the process is quite intuitive, the principles behind Extrude are fairly elaborated as discussed below:

• First, the algorithm determines the outside edge-loop of the extrude; that is, which among the selected edges will be changed into faces. By default (see below), the algorithm considers edges belonging to two or more selected faces as internal, and hence not part of the loop.
• The edges in the edge-loop are then changed into faces.
• If the edges in the edge-loop belong to only one face in the complete mesh, then all of the selected faces are duplicated and linked to the newly created faces. For example, rectangles will result in parallelepipeds during this stage.
• In other cases, the selected faces are linked to the newly created faces but not duplicated. This prevents undesired faces from being retained “inside” the resulting mesh. This distinction is extremely important since it ensures the construction of consistently coherent, closed volumes at all times when using Extrude.
• When extruding completely closed volumes (like e.g. a cube with all its six faces), extrusion results merely in a duplication, as the volume is duplicated, without any link the the original one.
• Edges not belonging to selected faces, which form an “open” edge-loop, are duplicated and a new face is created between the new edge and the original one.
• Single selected vertices which do not belong to selected edges are duplicated and a new edge is created between the two.

Depending on what is selected, and what select mode is active, the tool might pop-up an Extrude menu proposing you one or more of the following choices:

Region

Will perform the extrusion exactly as described above.

Individual Faces

Follow the algorithm described above, with one exception: “regions” of faces (i.e. groups of neighbor faces) are not extruded in a “block”, but each face individually (and by default along its own normal). This has several consequences: first, “internal” edges (i.e. edges between two selected faces) are no more deleted (but the original faces, yes). And FGons are “re-exploded” into their constituting triangles and quads…

Only Edges

No face nor vertex is extruded, only the selected edges are.

Only Vertices

No face nor edge is extruded, only the selected vertices are.

Grab mode is automatically started when the extrude algorithm terminates, so newly created faces, edges, and vertices can be moved around with the mouse. You can lock the extrusion along any global or local axis, as with a standard transform operation – when extruding faces, the movement is by default locked along the average normal of all extruded faces.

However, by hitting S during an extrusion, you can switch from Grab to Scale mode – the extruded vertex(-tices) will then be scaled toward/away from the current pivot point. The same goes if you want to do a “rotated extrusion” – hit E » R, and the newly extruded vertex(-tices) will rotate around the pivot point. Note that with the default Median Point pivot, when you extrude a single vertex, the pivot point is at the same position as the original vertex, so scaling/rotating the extruded one won’t have any effect, it will remain at this same position too!

Extrude is one of the most frequently used modeling tools in Blender. It’s simple, straightforward, and easy to use, yet very powerful. See this tutorial as an example.