What's the difference between a sofa bed and a sleeper sofa?

Separate mattress for folded bed, it is easy to understand that this is a sofa sleeper, because a sofa bed does not have a regular mattress inside it. 

Many sofa sleepers don’t have separate mattress but they have connected mattress or thick pad hided inside and it opens for body area, that means 

sleeping place is not the sitting place.

sofa beds are futons which close for sitting and lay flat for a sleep, that means the sleeping area is the same as sitting area.