A sofa is a long upholstered seat usually with arms and a back, often convertible into a bed

. It is more formal and has prominent backs and armrests, primarily meant for sitting

. The term "couch," on the other hand, is used to describe "a piece of furniture with no arms used for lying"

. In summary, a sofa is more formal and intended for sitting, while a couch is less formal and often used for lying down