.NFO is a commonly used three-letter filename extension of ASCII text files that accompany other files and contain information about them. Such NFO files can be viewed with text editors or dedicated NFO viewers. They commonly also contain elaborate ASCII art.
Different kinds of files with the same NFO filename extension on Microsoft Windows PCs are data files associated with a Microsoft software tool called System Information. Generally speaking, these kinds of files are less often being referred to when "NFO files" are being mentioned.