What is an ISO image? I can't find anything that defines
what it is, what it does, and how it's used.

http://www.iso.ch/iso/en/ISOOnline.openerpage

Quickie:
It's an image file containing many files, similar to a zip file. iso's are usually used to create bootable cd's.

Cool

Usually it's a copy of an entire CD in a file that can be used to pretty much exactly recreate the original CD.

# dd if=/dev/hdd of=filename

Would create an ISO image of /dev/hdd assuming that's the device of a CD-ROM to file "filename". Various Linux CD burning software can burn them back to a CD for you or even Windows apps like EZ CD Creator or NERO, etc... Generally this method copies tracks and sectors and cares not about filesystems. A Windows PC can copy a Linux CD this way even if it knows nothing about Linux (ISO 9660 CD format is compatible with both OSes).

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 )

More info at :
http://www.tldp.org/HOWTO/Filesystems-HOWTO.html#toc8

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.

OK, I get the idea. Thanks , all.