The world defines the environment that the scene is in. The surface shader sets the background and environment lighting, either as a fixed color, sky model or HDRI texture. With volume shaders the entire scene can be covered in mist or other volumetric effects.
For image based lighting, use the Environment Texture node rather than the Image Texture node for correct mapping. This supports Equirectangular (also known as latitude/longitude) for environment maps, and Mirror Ball mapping for converting photos of mirror balls to environment maps.
Currently this is most useful for night time or other dark scenes, as the world surface shader or sun lights will have no effect if a volume shader is used. This is because the world background is assumed to be infinitely far away, which is accurate enough for the sun for example. However, for modeling effects such as fog or atmospheric scattering, it is not a good assumption that the volume fills the entire space, as most of the distance between the sun and the earth is empty space. For such effects it is be better to create a volume object surrounding the scene. The size of this object will determine how much light is scattered or absorbed.