Jane,
ISO is the cd format that can be read by both Windows and Mac OS.
From OS 10.4 Mac Help:
"Creating a CD formatted for MS-DOS (Windows)
When you burn a CD in the Finder, Mac OS X creates a disc in "hybrid" format that can be read by most computers, including Windows computers.
Insert a blank CD or DVD disc into the optical drive of your computer.
If a dialog appears, choose Open Finder from the Action pop-up menu and type a name for the disc. Then click OK.
If no dialog appears, click the title of the disc icon when it appears in your Finder window, and type a new name for the disc.
Drag files and folders to the CD (double-click the CD icon to open its window and arrange the files exactly as you want them before burning the disc). The names of files, folders, and the disc can't be changed after the CD is burned.
Choose File > Burn Disc.
Choose the burn speed and click Burn.
You can also drag the disc icon to the Trash instead of choosing Burn Disc from the menu. The Trash icon changes to a burn icon and you see the same dialog to choose the disc speed.
The disc contains these filesystems: HFS+, ISO-9660 with Rock Ridge, and Joliet with Rock Ridge."