CDs and DVDs are two completely seperate and distinct pieces of technology but to the naked eye they look exactly the same.  The biggest difference that you will probably notice is that a DVD can hold far more data than a CD.

The big difference in the amount of data storage will become apparent when you understand how data is written to or burned to CDs and DVDs.  As the disc spins a laser is moved across the surface to burn pits in a spiral groove around the disc. A laser is an intensely focused beam of light and all lasers operate on a particular wave length. A smaller wave length will produce a smaller pit. A smaller pit obviously takes up less space and ultimately resulting in the ability to store more information in the same amount of surface area.

CDs and DVDs store everything, including audio and video, as a series of ones and zeros which means they are digital data storage mediums. Pits and lands (no pits) burned onto the surface of the disc are what make up the ones and zeros. The laser light will reflect off the lands but not off the pits when the disc is being read. Optical technology reads the data and converts into the ones and zeros that your computer can then understand.

The tracks of a DVD are narrower as well, which allows for more tracks per disc and translates into more capacity than a CD.  The common single layer DVD holds 4.5 GB of information, while a CD holds only 700 MB

Because a DVD has smaller pits and a laser needs to focus on them the physical make up of a DVD is different to a CD.  This is achieved by using a thinner plastic substrate than in a CD, which means that the laser needs to pass through a thinner layer, with less depth to reach the pits.

DVD technology also has a much faster rate of reading and processing data.  The difference in speed is huge with a 52X CD drive reading data at 8Mb a second and a DVD 24X drive reading data at 32MB second.

DVDs are the most popular choice for movies and data storage these days but are slowly being taken over by Blu-Ray. CDs are still very common and very cheap to purchase but I imagine they will slowly be phased out due to the increasing need for larger and larger amounts of storage space