What is an ".iso" file?

An .iso file is a type of archive file composed of the data content of an optical disc. The file name comes from the ISO 9660 file system that was used on earlier data CD-ROMs.

Files with the .iso extension are created by disc imaging software and are intended for use with writable discs such as CDs and DVDs, but can also be used for USB flash drives. The imaging software works by reading every sector of a disc in sequential order, so that when the .iso file is opened and used to write a disc, the sectors can be written in the same order and match the original layout.