From Valve Developer Community
- A normal is a float value between 0 and 1 that often represents a percentage.
- A normalised vector is one with a length of 1.
- A vertex normal defines the "front" of that vertex, determining among other things how much lighting it receives from a given angle.
- Normal mapping is a technique in which vertex normals are modified per-texel by a special texture, creating the impression of extra depth.