Living Room vs. Family Room?

Being a person always willing to highlight my real estate ignorance, this interesting article (and associated slideshow) from Slate on a town being built in a former cornfield outside of Philadelphia made me realize I don’t know the difference between a family room and a living room. Wikipedia says the difference is one of formality and highlights the fact that the differences are blurred for those of us who grew up in homes with only one such room. To confuse matters more, we always called our room the den, which (according to a wikipedia redirect) is the same thing as a study.