UV mapping is a 3D modelling process of making a 2D image representing a 3D model. The map transforms the 3D object onto an image known as a texture. In contrast to "X", "Y" and "Z", which are the coordinates for the original 3D object in the modelling space, "U" and "V" are the coordinates of the transformed object. Then the image is back-transformed ("wrapped" or "painted") onto the surface of the 3D object. For example, it is used to give an animated character a realistic looking face, but various other applications are possible.
When a model is created as a polygon mesh using a 3D modeler, UV coordinates can be generated for each vertex in the mesh. Then each triangle will map to the desired part of the texture, allowing a "decal sheet" technique to be used.