This article is from the CD-Recordable FAQ, by Andy McFadden (firstname.lastname@example.org) with numerous contributions by others.
Actually, .CDA files aren't really files at all. Windows shows the tracks
on an audio CD as ".CDA" files for convenience. For example, you can
create a file association for ".CDA" and invoke an audio CD player when
you double-click on a track.
The tracks themselves are in a format almost identical to a common WAV
or AIFF file. See section (2-20).