Welcome to Edit Mode

The last lesson dealt with objects in the Object Mode which is just one of six modes in Blender. Now, we will discuss the Edit Mode which allows us to manipulate individual vertices, edges, and faces of the mesh objects. For this example I will us a cube again. The cube mesh is made up of 8 vertices (in the cube's case, they are the 8 corners), the edges that connect them, and the faces that are made by these edges. In Object Mode, and when we render something, we do not see these because it renders (usually) just the face of the mesh. But now, in Edit Mode, we can see all these different parts and manipulate them to create more specific models.

First, add a cube to the scene if there isn't already one there and hit Tab (or go to the bottom bar of the 3D view and click on the menu that says "Object Mode" and select Edit Mode). When you do this, only the object currently selected will go into Edit Mode.

You will immediately see everything turn orange and the vertexes and edges will be "outlined" so they can easily be seen. Currently, everything is selected. So lets fix that by hitting the A Key to deselect everything (when nothing is selected, the A Key selects all). Now, right click on a single vertex to select it (it will turn orange). You can use the Move tool we learned about last time to move this vertex around, changing the overall shape of the cube. With that vertex selected, press the G Key and an axis key if you want to lock the movement to a particular axis. Now, move the mouse to move the vertex around:

You can also select multiple vertices, edges or faces and do the same thing:
(To select multiple vertices, select one then hold down Shift and right click on the other ones. If you select to vertices that are connected by and edge, the edge is essentially what you are selecting.)

Moving an edge.

Moving a face.

Technically, you can actually use all three transform tools on a vertex, but since a vertex is just a point and usually isn't rendered, rotating it or scaling it won't even affect it. However, if you are dealing with an edge or a face, all three transform tools do work. So let's try some funky scaling. Select an edge by holding down the Shift Key and right clicking on two consecutive vertices. Now hit S and scale the edge outward:

Spin the shape around to the opposite side by holding the Alt Key, left clicking and dragging to spin the view. First deselect everything with the A Key. Select the backside face (Shift Key and right click all four vertics). Hit S and scale down:

Now we have a pretty interesting looking shape. But say we want to rotate one of the edges on that front trapezoid looking side to make it look more like a parallelogram. Deselect everything with A. Spin back around to the front. Select an edge on either side, hit R and rotate it like so:

Now you can Tab out of Edit Mode back into Object Mode and BAM! All the changes are made to the shape! Make sure you get out of Edit Mode before working on anything else in the scene!

Hey wait! There's an extra goodie I picked out for you. This great video tutorial should help you out.
http://www.blendercookie.com/getting-started-with-blender-modeling/