A .ccd file is a CloneCD Control File. A CloneCD image would consist of three files, a .CCD file, a .IMG file and a .SUB file. The CCD file is simply an ASCII text file that holds information on the CD that is needed for the burning process.
A .ccd file on its own is useless, there also must be the .sub file and the .img file. These files must also be in the same directory (same folder on same hard disk drive) and must have the same file name. For example a folder like C:Images would have to have IMAGE.CCD, IMAGE.IMG and IMAGE.SUB so that it can be burned to disc or mounted onto a virtual drive.
There is a variety of software titles that can make use of .ccd files. For example, Alcohol 120% or Daemon Tools can mount CloneCD Image Files onto virtual CD drives (so you don't need to burn to use them) or you can burn the image with CloneCD.