Although there might be technical distinction recognized by furniture makers or some such, I've always heard and used davenport, couch and sofa interchangably. "Loveseat", however, is reserved for couch with only room for two.

My American Heritage Dictionary defines a "couch" as a sofa which a patient lies while undergoing psychoanalysis or psychiatric treatment (Wow! That's news to me! Although, calling the multiple-occupant upholstered seats in our house "couches" now make sooo much more sense.); it defines a "sofa" as a long upholstered seat, usually with a back and arms; it defines a "davenport" as a large sofa, often convertible to a bed, or a small desk.