ok a quick autopsy of a cd would probably help
there are 3 layers in total.
1- the substrate. This is made out of plastic and is injection moulded to get all the depressions in to it ( the music)
2- a layer of metal is electroplated on to the substrate. this metal is only 20-30 microns thick if i remember right.
3- a layer of laquer is layer on top of the metal layer ( this is also very thin but not quite micron level )
you then print on the laquer side.
the little ring in near the hub is called a stacking ring.
If you stack a cd on top of another one they will only tough on the stacking ring thereby protecting the data.
as for double sided DVD's they are a bit different.
Basic idea is u make 2 single sided DVD's and glue them together ( no i am not joking )
If you ahve any other questions just as and i willtry adn answer them