The direction of a face normal determines which side a particular triangle or quad is visible from. A face normal can be thought of as an imaginary line extending from the center of a face off into the direction that the triangle or quad is facing. A face's normal is perpendicular (at a 90 degree angle) to its front.

It is important to note that single-sided faces are only visible from one side. Double sided triangles don't really exist in OpenGL or Directx at the hardware level, but are rendered by doubling up two single sided triangles with their normals facing in the opposite directions.

Viewing face normals in Blender

1. Select your mesh and switch into edit mode.
2. On the Mesh Tools More or Mesh Tools 1 panel of the editing buttons (F9), click the Draw Normals button. Face normals are drawn as a blue-ish line extending from the center of the face:
   

Flipping face normals in Blender

Faces that are turned in the wrong direction can be flipped by reversing their face normals:

1. In edit mode, select the face(s) to be flipped
   
2. Press the 'w' key and select Flip Normals from the Specials menu that appears: 
   
   The selected face normals are now flipped:
   

Recalculating outside normals in Blender

Another method that can be used to correct the normals on faces that are turned in the wrong direction is Blender's "recalculate outside" feature. This method works well only when your mesh is closed (has a distinct inside area and outside area with no leaks between the two)

1. Select all faces in edit mode.
   
2. Press "ctrl-n" and choose Recalculate normals outside from the confirmation menu that appears.
   
   If all went well, the normals should now all be facing towards the outside of the mesh. If some of the faces are still facing the wrong way, they may be flipped manually using the instructions above.  Sometimes the recalculate normals feature leaves you in a worse position than when you started.  If this happens, the normal calculation may be undone by pressing ctrl-z.
   

See Also:

• Vertex Normals
• Double Sided Meshes
• Transparency and Drawing Order