Microsoft Excel is a commercial spreadsheet application written and distributed by Microsoft for Microsoft Windows and Mac OS X. Pre-2007 versions of Excel use XLS as the primary format for saving files. It features calculation, graphing tools, pivot tables, and a macro programming language called Visual Basic for Applications. It has been a very widely applied spreadsheet for these platforms, especially since version 5 in 1993, and it has almost completely replaced Lotus 1-2-3 as the industry standard for spreadsheets. Excel forms part of Microsoft Office. The current versions are 2010 for Microsoft Windows and 2011 for Mac OS X.
Microsoft Excel up until 2007 version used a proprietary binary file format called Binary Interchange File Format (BIFF) as its primary format. Used as the basis for XLS files it is a persistence format that supports authoring and manipulating content in workbooks and workbook templates. Most versions of Microsoft Excel can read CSV, DBF, SYLK, DIF, and other legacy formats.
Rich Text Format (RTF) is a file format that enables you to write a text file, save it on one operating system and then have the ability to open it on another. You cannot add images or objects - it is purely aimed at text. You can create a file using Microsoft Word in Windows, save it as an RTF file (it will have a .rtf file name suffix), and send it to someone who uses WordPerfect 6.0 on any version of Windows and they will be able to open the file and read it. Most people would have seen it when writing a 'wordpad rtf' file.
The RTF Specification uses the ANSI, PC-8, Macintosh, and IBM PC character sets. It defines control words and symbols that serve as common denominator formatting commands. When saving a file in the Rich Text Format, the file is processed by an RTF writer that converts the word processor's markup to the RTF language. When being read, the control words and symbols are processed by an RTF reader that converts the RTF language into formatting for the word processor that will display the document.