CD and DVD are basically the same but on the DVD data are written on a smaller scale, so they can hold a lot more, around 6 more times the data on a DVD. Also DVD can have double layer, it's one DVD data layer on top of the others on the same disc, the strengh of the laser is tweaked to reach the second layer passing through the first semi-transparent layer of data. Blu-ray disc can have 2 layer too, only written by a blue-violet laser instead of a red one, blue-violet have a shorter wavelength than red allow the disc to be written/read smaller data and hold around 6 more times than DVD. The HD DVD format who is discontinued was able to hold around 2 times the data a DVD can hold.
For the commercial CD and DVD you buy the data is pressed using a metal substrate deposited on polycarbonate and top-coated, put into 2 layer of plastic.
For the recordable CD and DVD it's a single plastic layer and the data is hold on the sticker they put on top of it. They put a empty data layer and the laser strength is increase to burn it. After the data layer. who previously reflect the laser everywhere, now it doesn't reflect on spot that where burn, making all the 0 and 1 for the data. CD-RW and DVD-RW, DVD+RW are the same only they have many data layer who can be burn instead of 1 for the CD-R, DVD-R, DVD+R. The focal of the laser adapt to the currently burn layer and can read data. With these type of optical device you can only burn a define number of times before you ran out of data layer to burn, making the rewritable disc read-only.
Actually there some software who allow you to burn image, gif, jpg on the data layer and you can see them, take a look at this :
http://www.instructables.com/id/Burning-visible-images-onto-CD-Rs-with -data-beta/
