CCD is the data information of the disk image file in CloneCD format. The CCD file contains information about the logical structure of the CD. It needs to be combined with an image file (usually with .img extension) to be burned. It may also come with a subchannel file (usually .sub).
The .ccd extension can be sued directly by first-party disc emulator Virtual CloneDrive. It can also be mounted with thir-party virtual drives such as Daemon Tools and Alcohhol 120%. The command line application ccd2 is also available to convert ISO9660-compliant ccd/img to an ISO image.