A family of Microsoft word processing software products for creating web, email, and print documents.
Plain text can be read by pretty well any text file editor. There are no supported text formatting attributes like bold and italic. It represents the lowest common denominator for simple text only "documents"
Rich Text Format, RTF, supports some limited formatting, like bold and italics. It represents the lowest common denominator for "word processing" programs. Many, most(?), word processor programs will read and edit RTF format files.