What are the planes?

In order to have a discussion

According to
http://www.dandwiki.com/

The planes of existence are different realities with interwoven connections. Except for rare linking points, each plane is effectively its own universe with its own natural laws,

The planes break down into a number of general types: the Material Plane, the Transitive Planes, the Inner Planes, the Outer Planes, and the demiplanes. 

 While I think this may be a good start, I don't think it's good enough, even defining what each of these things mean.

Any thoughts?