An (or ISO image) is an archive file that contains an exact copy of the data from an optical disc, such as a DVD. A Windows 7 ISO is a digital replica of the installation DVD for Microsoft’s Windows 7 operating system. It contains all the system files needed to install, repair, or reinstall Windows 7 on a compatible computer.
Installing Windows 7 today is the easy part; getting it working is harder due to the age of the software.
If you are looking for an older build or if the Microsoft servers are unresponsive, the Internet Archive often hosts backups of official ISOs.
Even if the ISO appears to work, you cannot trust its integrity. Always verify file hashes (SHA-1, MD5) against official Microsoft values if possible.