Hi BruceM,
There is no significant difference in the file format between Office 2007 and Office 2010. So it's no problem open a file from Office 2010 in Office 2007.
But it doesn't mean there is no difference between Office 2007 and Office 2010. You can refer to this article to get more information about the difference between Office 2007 and Office 2010:
http://www.slideshare.net/GurpreetSingh160/ms-office-2010-vs-2007
Please Note: Since the web site is not hosted by Microsoft, the link may change without notice. Microsoft does not guarantee the accuracy of this information.
Hope it's helpful.
Regards,
Emi Zhang
TechNet Community Su