If you would like to know more about the inner workings of how textures are mapped to a 3D surface, you can read the information available at http://developer.nvidia.com/CgTutorial/cg_tutorial_chapter03.html. For a complete list of the options available when importing a 2D texture, you can refer to the following website: http://docs.unity3d.com/Manual/class-TextureImporter.html