The origin of the image is very often a photograph of a natural surface. Here, a simple cube is texture mapped with a photograph of a brick wall. The practical importance of this kind of texture mapping cannot be overemphasized. Many objects with simple geometries become completely convincing when mapped with photographs of realistic surfaces. But such photorealistic surfaces can take on completely new value when used in unexpected ways, as on text.
Texture maps can be decorative rather than realistic.
A simple repeating geometric pattern can make an outstanding texture map. Note how interesting and sophisticated the simple text looks after the application of the map. Here's the same texture map on a sphere. The entire issue of how a flat image can mapped around varying types of geometries is a big one that we will address in future lessons. A simple comparison of the last two images, though, will alert you to both the obvious difficulties and artistic opportunities involved.