The universal standard filesystem for data CDs.
The word ISO is commonly used to refer to the iso9660 standard. Different vendors have implemented extensions to the standard ( High Sierra, Rock Ridge, MS Joliet to name a few )
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The best way to store a CD filesystem in your hard-disk is to make an iso image of it ( one file - max size 670 MB or so ) . This makes it convenient to lay it on a fresh CDROM should the need arise. Most Linux installations are distributed as ISO images that you can download and burn to CDs.