CDs are items that can be found on Earth containing specific data.
They can be inserted in any terminals, such as Lyoko's Terminal or laptops.
In the case of Lyoko's Terminal, you must right-click the CD drive to insert/eject a CD while it is in your hand. As for laptops, you can simply right-click the laptop with the CD to insert and use the “Eject CD” app to eject it.
Music CDs are packaged with the Music Player program, allowing you to listen to the music burnt on the CD.
Such CDs can not be located with the Kadikeon program and must be found through exploration.
Franz Hopper's Diary has two different CDs: the encrypted CD and the decrypted CD.
You can learn how to obtain these CDs by reading the page dedicated to the Diary Mission.
The encrypted CD allows you to read the reports that explain how Lyoko, XANA and the Towers function while the decrypted CD allows you to read the story entries that explain Franz Hopper's narrative.