Conditional division to Front and back Sides of the Face is necessary for different purposes: modeling automation(plug-ins), texturing, rendering and other engines. Do not be surprised when you`ll have any problems if you ignore the plane orientation.
1. The basic way to change face orientation is to klick on it with right mouse button, and find Reverse Faces in context menu.
2. Little more advanced way.
3. And this is a magical tool “ReverseOrientFaces” by Fredo.