Microsoft Office 2007, 2010, 2013, 2016 and Office 365
A NOTE ON OFFICE 2016: It is the end of September and Microsoft has released Office 2016. I am trying to install it now and am having some issues (Windows 10 upgraded from Windows 8.1, no previous Office installation). The install gets as far as 90% and seems to freeze there (it seems to have something to do with the Office Click-to-run). I am able to run the applications, but trying another install failed because the current one didn't finish. I managed to get past it, but I fear that this will be an issue for many (my system was pretty clean, others won't be so lucky). I have not tried other Office versions on this system.
All of these versions are variations of the same theme. Gone are the toolbars and menus of Office 2003 and earlier, replaced by the "Ribbon." The Ribbon contains the functions that pertain to what you are doing at the time. If you are typing, the ribbon has all of the text formatting functions at your fingertips. Insert a picture, and the ribbon changes to picture formatting. Highlight some text, and right nest to your curser, a small text formatting function appears, so you don't have to move you mouse up to the toolbar to change the text (a very handy thing if you so this a lot). You can also preview style changes to text just by rolling your mouse over the style examples, without having to commit the change, decide if you like it and undo the change if you don't. There are also many more advantages to this new user interface, too many to discuss here. In addition, these newer versions are able to save their files as PDF files.
Office 365 is something different. Although the functionality is the same as Office 2013 (or 2016), Office 365 is a subscription service. The price varies depending on what your needs are, but runs about $100 per year for a home subscription for up to 5 PC's/Macs/Tablets, and $60 per year per device for businesses.
Each version has tweaks, not major changes. If you get used to Office 2007, then 2010, 2013, or 2016 will not be an issue. Office 2013 and 2016 is designed with Windows 8 in mind, so it is uglier than the earlier versions.