For Ubuntu, the CD still serves as a common installation medium. Other uses of CDs include storing multimedia, such as music or movies, storing software or backing up data.
CDs typically store up to 700 MB of data. Most CDs are not writable, they can only be read, but CD-RW discs are rewritable and can be written to several times.
As data sizes grow larger, CDs are being replaced in many situations by DVDs (which hold over six times as much data). However, CDs are still prominent in many areas, are still cheaper and are still more likely to be readable by a given computer.