Why is a .ISO called a .ISO? [closed]

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?

From the ISO 9660 standard.

But it's just a way to know what a file is about.

The origin is the ISO 9660 file system which is used on CD-ROM. The extension .iso remained also for DVDs and Blu-Rays which use the UDF (ISO/IEC 13346) file system. See here for details.

The name is derivated from a norm issued by the International Organization for Standardization which specifies the file system on an optical medium [1]. That norm has the abbreviation ISO 9660 [2] and you can guess now why a CD-ROM image (and later on a DVD-ROM image) usually is named .iso.

[1] http://en.wikipedia.org/wiki/ISO_9660

[2] 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.