1) On a mass produced DVD Video, there are tiny pits and smooth areas - the pits reflect the laser light away from the read sensor to mean "0", and the smooth areas reflect the light on to the sensor to mean "1".  These DVDs are called "pitted" or "pressed" DVDs. These DVD are made in very large quantities where protective polymers layers are "pressed" onto the "pitted" layers .  A DVD from the video store is an example of these DVDs. These DVD have a longevity span of decades.

2) On a computer writable DVD (DVD-/+R) , there is a very thin layer of organic metal alloy dye build into the DVD. When writing to the DVD, the laser burns tiny holes in the dye layer or discolors the dye layer (much like exposing movie film to light). These variations color simulate the "pits" on a pressed DVD (see #1 above).  Because this dye layer is light and heat sensitive, both heat and light from subsequent laser reads and light and heat from the environment can continue to break down the dye layer.  Over time, 2 to 5 years, after the initial burn, serious digital data loss and observable digital image loss can result. These DVDs hold roughly half the data (4.7 Gigabytes) of a pressed DVD. These DVDs  should only be used as temporary media.