What are the differences in the structure of a CD, CD-R, CD-RW, DVD and Blue-ray?
CDs have the data layer on the surface of the disc, DVDs (and I think Blu-Ray) have it in the middle. The reflective layer is applied to the back of the data layer. A CD has a protective layer on top of the protective layer, then a screened label. A DVD is two pieces sandwiched together. A writable disc has a mostly empty track formed in the polycarbonate, with a dye applied before the reflective layer. The dye is as such as its optical state can be changed with a powerful enough laser. Re-Writable discs have a dye that can be reset by heating it to a lower temperature, by laser. CDs/DVDs/Blue-Ray all have different track/pit densities, allowing for more capacity, because the shorter wavelength lasers can read finer pits/tracks.
