I faced this challenge, this evening, as I taught my children how to install Windows XP onto their computers. (Part of our homeschool computer class)
My answer: a radio button is a round button you can click on to select one of several choices in a list. You can only select one radio button at a time. When you select another button, the original button is deselected - it "pops up," just like the mechanical car radios. (Unless, of course, you were like me as a kid: I tried to see how many buttons you could push down at the same time.)
Google, provides a technical definition here.
However, Jake Howlett, has a much better explanation.