The only difference between a pressed CD and DVD is the size of the bumps/pits, depending on which side you look.
If you are comparing a CDR to a DVDR then the major difference is the photosenstive dye used. When the high power mode laser is writing to the disk the dye reacts and turns dark. When the low power mode laser is reading the light reflects back where it isn't dark and and doesn't reflect back where it is.
Finally the difference between a CDRW and DVDRW is the active alloy used. In high power mode the laser basically melts the alloy in a specific region and when the cyrstalline stucture cools off it loses its reflectivity. In a lower power mode the crystalline stucture can be reformed to become reflective again.
DVD+RW and DVD-RAM use a high frequency wobbled grove technique while the DVD-RW uses a lower frequency wobble. In a nutshell the high frequency wobble allows for better data integrity. The +RW and -RAM also have better error correction built in.