A .ISO is a digital copy of a cd or dvd. Almost 90% of all digital copys end on ".ISO". Why is this? Was there someone that called it .ISO and everyone just copied it?
The name is derivated from a norm issued by the International Organization for Standardization which specifies the file system on an optical medium . That norm has the abbreviation ISO 9660  and you can guess now why a CD-ROM image (and later on a DVD-ROM image) usually is named
 Some confusion can arise why ISO isn't an acronym of International Organization for Standardization. Quoting once again Wikipedia:
Recognizing that its initials would be different in different languages, the organization adopted ISO, based on the Greek word isos (ἴσος, meaning equal), as the universal short form of its name.
.iso extension is an alternate or a shortened form of
.isoimg, which could stand for "ISO 9660-compliant disk image."
The original ISO 9660 standard appears to have been produced in 1988, when PCs were still using 8.3-character filenames (e.g. names could be only 8 characters long, and extensions only 3), so ISO was likely the first choice for shortening the extension to three characters.