I like the word “sofa” better than the word “couch.” Sometimes I catch myself using the latter term, however.
I think of window treatments as blinds, shades and other things cover just the window pane. Curtains and drapes are cloth, window treatments are not. I could get more finicky and say that curtains open and close while drapes are purely decorative, but that distinction is less concrete in my head.
Interchangeable.
Lounge? Over here that means living room.
Blimey. If we were told to go and sit on the lounge divan in Chesterfield, we would all now probably just decide to stay standing.
Same. And I’m also from Sydney.
There’s also this for people that had one for guests (that you hate). The pull out. You can sleep on our pull out in the living room.
dictionary says “divan” is a couch. we used to call it that when we were trying to be “cosmo” lol (cosmopolitan)…like describing front-door entryway as a “foyer” in a four room house…also Daddy said “mounting” one time for “mountain” when some big-wig came by (can you tell we’re from the country?)
We live in all our rooms. 
What you call a living room, we refer to as a lounge room and, by damn, lounge around I do!
I don’t know, sometimes I feel like I’m dying if I go into the bathroom too soon after my husband or my son…
couch is redneck - sofa is high class.
same piece of furniture for the majority of cases - loveseat is the third piece in a set - not quite chair, not quite sofa.
If you want to use them to mean different things, there are definitions that lend themselves to this. A couch was originally something to lie on, and can reasonably be restricted to mean either a psychiatrist’s couch or a piece of living room furniture that’s long enough to stretch out on. A sofa was originally a cushion- or rug-covered dais, and it’s probably the best word for the general class of two-armed upholstered furniture that’s longer than a chair. I grew up using them interchangeably, but it’s tempting to adopt different meanings for them.
A chaise longue was originally a one-armed chair that one sat on with the legs to the side, Roman-fashion, but no one does that anymore. The item of patio furniture that’s usually called by this name is really an outdoor couch, but no one calls it that. Loveseat is pretty universal in meaning a short sofa for two, though many “loveseats” I’ve seen are big enough for three lovers. 
A divan was originally a backless sofa, with or without arms, that one put against a wall, or a bureau or other furniture; only antiques and faux antiques are made that way nowadays, so it’s another word that could be used for sofa, but really – why? Davenport and Chesterfield were specific brands once, and can still be specific styles if you don’t mind the confusion among people who use them generically. I never heard these when I was growing up, except “Davenport” in movies, and “Chesterfield” as a brand of cigarettes.
“Window treatment” is the whole package. Several of the treatments pictured include blinds, and a couple don’t include drapes. Using it as a synonym for “drapes” is inaccurate and pretentious.
Jokes about the Empire aside, what exactly is the confusion over “ottoman”?
reported.