What does extension .xlsx mean

I have just received an eMail from my accountant and here file name extension is .xlsx

I have never seen this before. Can someone tell me why the x on the end of the extension?

Also, when it opens, a Message Box is displayed: File Conversion in process.

Its an Excel 2007 file which uses a different format to earlier versions which is why you get the File Conversion in process message when opening the file

Thanks Mudraker . . .

One more question. I have a program that I have developed in Excel 2002. If a user has Excel 2007, will the macros run OK ? I've heard it may be difficult to go backwards, but forwards is OK. Is this true.

I don't have any experience with 2007

Newer versions of Excel VBA have commands that are not in the older versioon so the theory is that if you want a macro to run on different versions of Excel built it on the oldest version and it should work in the later versions

The main thing that crops up with VBA in earlier versions failing in 2007 relates to the FileSearch method no longer being available (and the ribbon) - for the vast majority of code there shouldn't be any issues.

Just to add: .xlsx is a VBA free 2007 file, .xlsm is a 2007 file with VBA embedded ... the 2007 files have better compression than earlier versions... your accountant can save the 2007 file as a 2003 file type before sending to you if preferred.