The extension .ISZ refers to files of the type, Zipped ISO disk image files. The ISO files are nothing but an exact replica of an optical disk such as a CD or DVD. The ISZ file is the compressed and zipped version of an ISO file.

The file structure of .ISZ was developed by ESB systems, and the architecture of the file is similar to an ordinary ISO image file. The best feature of an ISZ image file is the ability to compress the image data without losing an metadeta info thus allowing to store the exact copy of an optical disk in a soft copy which occupies lesser space on the hard drive.