The extension usually indicates the ISO file containing a ISO9660 filesystem that corresponds to the contents of a CD...
you can read the content of the image file with some commands, for example, proceed as follows:
mkdir /tmp/MyCDISOImage mount -t iso9660 -o loop MyCDISOImage.iso /tmp/MyCDISOImage
we can now browse the content of the image file as well as normally do to access the files in any directory of the system:
ls /tmp/MyCDISOImage
when we finished reading the contents of the ISO file "umount" the image file as follows:
umount /tmp/MyCDISOImage rmdir /tmp/MyCDISOImage
