It’s all well and good to be able to render shapes in space in a photorealistic way, but at some point you’d like to draw something that doesn’t have just one surface color. After all, a billboard isn’t just a bunch of shapes each of which has one color – it’s one object with paint / ink placed on it in an ordered way.
Texturing accomplishes by taking a primitive shape (like a sphere, triangle, surface of revolution, etc.) and wrapping an image onto and over it. Let’s consider a sphere in space:
A white sphere in space.
Now let’s say we mean it to be Earth. Then we can take a picture of Earth that’s flat:
Flattened map of Earth
and then map it onto a sphere to get a picture of what we all know Earth to look like:
Behold!
Two Mars globes where the left is what is seen with the eye, and the right is a topographic map.