ISO refers to "International Organization for Standardization", but it really doesn't
stand for anything, it just means a collection of data that can be burned to CD or DVD. Within Linux it usually
means it will create a bootable Linux disk or installation disk. So "downloading an ISO" means you are downloading
a Linux distribution or installation CD.
Linux Online maintains a
large database of Linux distributions with a short
description of each. LinuxISO.org offers free downloads of
nearly every major Linux ISO. LinuxISO.org also has a lot of information to help you decide which
distribution is right for you.