Actually no, the terms are not interchangeable. Engine = liquid fuel, motor = solid fuel (although I suspect the actual difference to be more in line with what PEH mentioned, as opposed to being based on the propellant type used). For example, if someone talks about the space shuttle boosters, they will refer to them as "motors," not engines. OTOH, if they were talking about the propulsion systems mounted on the orbiter itself, they will use the term "engine," not motor.

Of course, to most normal people, this is all just semantics.