A CD Audio Track or .cda file is a small (44 bytes) file generated by Microsoft Windows for each track on an audio CD (it is a virtual file that can be read by Windows software, but is not actually present on the CD audio media). The file contains indexing information that programs can use to play or rip the disc. The file is given a name in the format Track##.cda.
The .cda files do not contain the actual PCM wave data, but instead tell where on the disc the track starts and stops. If the file is “copied” from the CD-ROM to the computer, it becomes useless, since it is only a shortcut to part of the disc. However, some audio editing and CD creation programs will, from the user's perspective, load .cda files as though they are actual audio data files.