Normal
Normal is a vector that represents a direction perpendicular to face or vertex. Actually vertices are points, and nothing can be perpendicular to point, so it just shows where does this point "look" in space, this information is used to calculate lighting of the face.
Normals are often used to produce smoothing effect and in normal mapping.
The length of face normal is always = 1.0 units. It is a unit vector.
The direction of face normal is perpendicular to the face itself. In normal mapping this direction specifies how the reflected light should behave inside of single face.