The world buttons let you set up the shading of your scene in general. It can provide ambient color, and special effects such as mist, but a very common use of a World is to shade a background color. These are accessible via the World tab.
While these world settings offer a simple way of adding effects to a scene, compositing nodes are often preferred, though more complex to master, for the additional control and options they offer. For example, filtering the Z value (distance from camera) or normals (direction of surfaces) through compositing nodes can further increase the depth and spatial clarity of a scene.