Textures are images applied to 3D objects. In games, most of the texture sizes are a "power of two", so typical sizes are 256 x 256, 512 x 512, or 1024 x 1024 pixels.
In modern games, there are several types of textures that affect the look of the model in the engine. The idea behind having that many textures in games is, like always, the need for saving performance. The following three are the most commonly used types:
Color/diffuse maps are the most basic textures determining the base color of the object, most of the time ignoring any lighting direction. This is what we did by polypainting the tree at the beginning of the book.
Normal maps are used to simulate high polygon details on a low polygon model by dynamically simulating light and shadow on the surface. You've probably seen them before because they can easily be spotted by the dominance of purple colors. Most of the time, normal maps are generated by software, opposed to handmade color and specular maps.
Specular...