A question came up today from a Macintosh user, asking why her professor couldn't read an Office 2008 file she sent.
We installed the Microsoft Office Conversion Pack back in the fall, so the professor's computer should have been able to read the file. But, after a couple of questions, I discovered the issue wasn't the file format itself, but rather the name of the file.
Windows computers use file extensions to identify a file. This dates back from the early DOS days, when you were restricted to the 8.3 format: eight character names (maximum -- it could be less) with a three-character extension, separated by a dot. Thus, you'd name a file "file.txt." The various extensions indicated the type of file: .wks for Lotus, .dbf for dBase, .ws for Wordstar, etc. (Word Perfect didn't automatically attach an extension).
When Windows was developed, Microsoft set up a scheme where a file was identified by its extension. If it matched one of the programs installed on your computer, you could double click on the file and open it.
Macintosh took a different route. The files were read by the system and identified so they could open with a click.
I'll admit I'm partial to the Windows system. It allows you to do things like change an extension from .csv (which is read in Excel) to .txt (which is read by a text editor) and then tweak the data. This is useful when the output of a database needs to be fixed.
In any case, Macintosh users are not used to adding extensions (they aren't done automatically like in Windows). If you're sending a file to a PC user, just give it an extension: .doc for Word, .xls for Excel, etc. Your recipient will be able to open it without making changes to the filename.