What is ISO image?
You need to have CD-ROM, DVD ROM to access data and executable from disc. With ISO image, you can create image of data from disc along with disc file system. Unlike general disc you don't need Disc reader to run, open content of ISO image. You can say your disc is read by a virtual disc reader. ISO image file having .iso extension
Some companies provide PC with no DISC reader to their employees, for data piracy protection. In such cases if system administrator wants to install some anti virus or say MS Office package, then he/ she will create Image of that anti virus software of MS office pack, and he/ she will use it for installation.
ISO images are also used for Games also. Earlier there were some games where they need to be read from Disc reader only.