Design Dictionary: What’s the Difference Between a Couch and a Sofa?

Here on Apartment Therapy, we tend to use «sofa» and «couch» interchangeably to refer to the place where you might entertain friends or plop down with a blanket for a Netflix marathon. But do the two words really mean the same thing? Where did they come from? And what, then, are a settee and a loveseat?