Displaying the normal does not affect the model, but sometimes it can help you in your modeling to see which way your faces are pointing:
Press Ctrl + MMB and use the mouse to zoom out so that you can see the whole cube.
In the 3D View, press N to get the Properties Panel.
Scroll down in the Properties Panel until you get to the Mesh Display subpanel.
Go down to where it says Normals.
There are two buttons like the edge select and face select buttons in the 3D View header. Click on the button with a cube and an orange rhomboid, as outlined in the next screenshot, the Face Select button, to choose selecting the normals of the faces.
Beside the Face Select button, there is a place where you can adjust the displayed size of the normal, as shown in the following screenshot. The displayed normals are the blue lines. Set Normals Size to
0.8. In the following image, I used the cube as it was just before you made the last extrusion so that it displays the normals a...