CD is used to store some data, which can be voice, pictures
and typed data. DVD is used to retrive the data from the
CD and can listern the music or view Picture based on the
data availabe in the disk.
The wavelength of the laser is reduced(from the 780mm
infrared light used in the CD) to 625mm cos of which DVD
technology has managed to write in smaller ‘pits’ as
compared to the standard CD. This allows for a greater
amount of data per track.The tracks of DVDs are narrower
which allows more tracks per disc this again translates into
more capacity than a CD.