Student or Learner
First of all, I live in a country where there are 3 seasons: hot, very hot and too (damn) hot. So I don't use them.
I thought that heaters and radiators were the same thing, just different names used in different places (UK VS US). Recently I have come cross a debate between a heater and a radiator as to which is more economical. So they are two different things?
What do you call this?
The technical difference aside, do people generally use these two words interchangeably?
We don't use them interchangeably.
Your picture shows a radiator. It's filled with water heated by a boiler and circulated by pipework through several in the building. It's part of a central heating system.
Click here to see what we call heaters. You'll observe that some of them look like radiators but they are designed to be moved from room to room.