It will depend on the specific type of file and version of Excel. The standard spreadsheet uses xls for versions up to 2003 and xlsx from version 2007. You can also have other types of files, like Excel template files, which would be xlt, or xlm for Excel macro files. There are several other types too.
Microsoft Word files are saved with the extension .doc
Microsoft Excel can open Microsoft Works files saved in the Microsoft Works 2.0 (or earlier) format. To open Microsoft Works 3.0 or 4.0 files in Microsoft Excel, you must first save them in a compatible file format.
.docx is the extension for Word 2007 and 2010.
Microsoft Excel was the software that effected by the virus. The virus is stored in a hidden datasheet, and effected all the files saved and accessed.
.htm and .html
Yes, you can use files saved on Microsoft PowerPoint 2003 in Microsoft PowerPoint 2007 (but not the other way around).
The files are saved on your hard drive or on whatever storage medium you select. Excel is very obedient and will put the files where ever you tell it (even if you don't remember where you told it to put the files).
You can open these files with Babylon electronic dictionaries software. See www.babylon.com. Microsoft Flight Simulator also writes saved games to the files with such extensions.