A scene graph is the structure of the world being created. The VRML file format allows you to create a scene graph using words and punctuation. This is why you can create VRML scene graphs using a simple text editor like HTML.
The scene graph is made up of a group of objects that when rendered exist in 3D space. VRML has many object types. The simplest are known as "primitive objects" because they can be described in simple terms. These primitive objects include box, cone, cylinder and sphere. These can be used together to make more complex objects.
VRML is made up of more than 60 node types. A "node" is a piece that implements some functionality. These nodes fit together in such a way that some have other nodes below them. The node above is called the "parent" node and the node below is called the "children" node. The relationship between parent nodes and children nodes is called the "scene graph hiearchy".
Within each node is a list of "fields" and "events" which hold values that define the parameters for that nodes function.
For example: The fields for the cone node define the height of the cone and the radius at its base.
VRML has three types of nodes:

1. Grouping nodes which contain a list of other nodes.
2. Children nodes which can be contained in the children lists.
3. Attribute nodes which are contained in a specific field of a specific node. They add attributes to shapes, sounds, etc.