Technically, there isn’t one. It’s a marketing thing.
camelback sofa: a loveseat, a sofa that seats two people.
chaise longue: a reclining chair with a lengthened seat forming a leg rest.
chesterfield: a sofa with padded arms and back of the same height and curved outward at the top.
button-back sofa: a futon (a Japanese quilted mattress rolled out on the floor for use as a bed).
davenport: a large upholstered sofa, typically able to be converted into a bed.
canapé: a small piece of bread or pastry with a savory topping, often served with drinks at a reception or formal party. Umm, or a pullout sofa.
daybed: a couch that can be made up into a bed.
divan: an S-shaped sofa on which two people can sit face to face, or a long low sofa without a back or arms, typically placed against a wall.