What are "Forms" in Microsoft Word 2010?

I'm trying to add a legacy ActiveX control (a radio button) to a Word Document in Microsoft Office 2010, but when I select it from the Controls section of the Developer tab, I am prompted with this message:

The program used to create this object is Forms. That program is either not installed on your computer or is not responding. To edit this object, install Forms or ensure that any dialog boxes in Forms are closed.

This sounds like a sensible requirement to me, but I can't find this "Forms" program anywhere on my computer or online... not even a mention of it. Microsoft's site and Google both have not yielded any fruitful results.

What is this "Forms" program?