Couches are furniture with a backrest and armrests that can easily seat 2-3 people.
Sofas are much larger than couches and can usually seat up to 4 people.
Settees are shorter and smaller than sofas and couches are best suited for small spaces.
Divans are basically mattresses on an elevated platform and are commonly found in Indian households.
An ottoman does not have a backrest; it is usually used as a stool or a footstool.