In computing, "plain text" is a term used for an unformatted readable text file that doesn't require much processing. There are no graphics or spell checker tools, just plain text.
The purpose of using plain text is primarily a "lowest common denominator" independence from programs that require their very own special encoding or formatting. Plain text files can be opened, read, and edited with text editors, for example Notepad (Windows), edlin (DOS), MS-DOS Editor, ed/vi/Emacs (Unix, Linux, and elsewhere), pico, nano, SimpleText (Mac OS), or TextEdit (Mac OS X).
Web developers tend to use plain text editors out of habit because of the lack of gremlins. Plain text editors are also preferred when filling out forms on the Web, in fact, we highly suggest that you cut-and-paste your text from an online resume or college application into a Word document to spell check it and make corrections, then cut-and-paste it to your plain text editor and turn the word wrap off, and then cut-and-paste it back to the Web site and review it for any formatting inconsistencies.