NFO is a variety of text file used for the specific purpose of identifying a downloadable software package, generally in the context of the "warez scene" where such files accompanied pirated software, especially on BBSs.
There were no specific formatting rules, but the files were generally in the MS-DOS/IBM PC code page 437, often using the various "extended ASCII" graphical and box-drawing characters for (extended) ASCII Art. Sometimes ANSI Art was used as well. The files generally expected the use of a fixed-width character set on an 80-column display, as was normal for PCs of the era in which it began use.
Viewing these files on modern systems can result in awkwardness, as the current-day character encodings differ from CP 437, and many characters come out wrong; also, use of a variable-width font causes art to fail to line up. Special viewers have been developed to handle these files.