Game Development Dictionary
A scene graph is a tree where the nodes are objects in a scene arranged in some sort of hirearchy. These nodes may be actual physical objects, or simply 'abstract' objects. For example a transformation node would apply some form of transformation to any 3D objects that are below the transformation node in the scene graph. A scene graph can be used for many things, depending on the way you order the nodes in the graph. For example you could have an octree containing object to be rendered in a scene, this would be a limited form of scene graph. You could have a scene graph that contains an octree as well as an alternative way or organising the same data, e.g. by render state. So you could use your scene graph for culling unseen objects as well as ordering objects to be rendered by render state.