The Scene Graph
You may need to read the article about transformation before this one.
A scene graph is a general data structure that arranges the logical representation of a graphical scene. It is a collection of nodes in a tree structure, with each node having one parent and a non-negative number of children. Only the top node has no parent.
Each node's transformation matrix is multiplied by all of its ancestors' to generate its world matrix, which represents its actual position in a scene relative to the origin. The transformation matrix represents its position relative to its parent.
The next article is about 3D.