I've seen the term
Normal being used in many places, but I'm not quite sure what it is or what it's used for; what exactly are they?
The Normals are usually used to determine a surface's orientation toward a light source for shading. Most common examples:
Flat, Goraud, Phong shading
Fresnel, Refraction, Reflection effects
- Bump, Normal, Displacement mapping
Normals are generally normalized to length 1 (in arbitrary units), otherwise calculation errors, and thus artifacts, may arise. Since they only point one way they are also used for back visibility culling (when double-sided is off).
That's why it is important for the normals to be consistent and coherent (i.e. point in the correct way - outwards of a model).
Besides the mathematical explanations, I think a picture will help to understand what is meant by normals and perpendicular. The faces having a normal (blue lines) pointing to a light source are rendered brighter than the faces pointing into other directions.
You can toggle the display of normals in the Properties Panel (right panel N):