Tangent-Space vs. Object-Space. Normal Maps
Normal maps come with two flavor, object-space and tangent space.
Object Space Normal
Tangent Space Normal
Object space normal is unique-per-mesh, and is not reusable.
Tangent space normal, by contrast, is easily reusable and great for deforming meshes, like characters, animals, flags, etc. It also works better with image compression (because most of the area is blue).