How Unwrapping and UVs Work
There’s something tricky that happens with textures, which define the colors of your models’ surfaces: You have a 3D model, but the image texture is 2D, so how do you “paint” a 3D model with a 2D texture? The answer is UVs. The 3D model has its vertices located on the X, Y, and Z axes, but for Blender, internally, they’re on the U and V axes as well, and the U and V axes are 2D positions, made to be used for texture projection. In the UV/Image Editor, you can access those UVs and adjust them to define how a texture is projected onto a 3D model.
Unwrapping (also called UV mapping) is the process of adjusting an object’s UVs so the texture projection works properly. It’s probably easier to understand if you see how this process works.
Do you remember those exercises when you were a kid in which you took a two-dimensional piece of paper and, by cutting and folding it in a certain way, you ended up with a three-dimensional shape like a cube? Unwrapping can be explained with that example, only in reverse order: you have a 3D model, you unfold it into a 2D shape using the UV/Image Editor, and then convert it into a plane. (It won’t affect the shapes in your 3D model at all; this only happens “under the hood.”) Figure 8.1 illustrates this procedure.
As you can see, unwrapping is like unfolding the 3D model and converting it to a 2D mesh. You can project an image onto that mesh and that projection will have effects on the 3D model itself.