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Bumpmaps

Surface Normals – a vector normal (perpendicular) to a point on a surface - http://en.wikipedia.org/wiki/Surface_normal

The Surface Normal decides the reflected direction of any light source and therefore the position of highlights and shadows.

The normals on a smooth, though not necessarily flat surface, stay normal to the surface.
The normals on a textured surface point all over the place to follow the tiny surfaces which generate the texture, this in turn changes the reflected direction of the any light and is why we perceive a textured surface.

In 3D modelling, and therefore in rendering, we do not try and model a textured surface, we imitate it using Bumpmaps.  The bumpmap is a representation of the texture which adjusts the normals across the surface.  When the renderer calculates the lighting this gives the impression of the texture through the highlights and shadows.

Bumpmaps are commonly a simple grayscale image.  The surface normals on the uniform surface are distorted gradually from the light to the dark areas of the image.

Bumpmaps can be found in various places or you can experiment with creating your own in PhotoShop