Simplifying, X3D (and it's older version, VRML) is a file format for 3D models. You will find that virtually any 3D modeling program can export to it, for example Blender includes an X3D exporter (see also our Blender exporting notes).
As a 3D file format, X3D is quite unique, as
It's not only a file format. It's actually a very flexible scene graph for 3D applications. Every X3D node corresponds to a Pascal class with appropriate fields, and you can freely create and modify X3D nodes at runtime.
It's designed to describe virtual 3D worlds, not just static scenes. So you can express animations, interactive behaviors (e.g. open the door when user presses a handle), and scripting right inside the X3D file. Many advanced graphic effects are also possible, like mirrors by generated cube map textures, screen effects, shadow maps, shadow volumes, effects using GLSL shaders and much more.
Learning X3D: Use this part of the documentation to learn about the available X3D nodes and their fields.
The nodes in the official X3D specification are divided into components. We list all the supported nodes, with links to their X3D specification. For some nodes, we also mention eventual caveats or simple extensions that we have implemented.
The unofficial nodes that we add documents some cool graphic effects available in our engine through special X3D nodes.
In any case, the X3D specifications are your ultimate resource to learn what you can do with X3D.