What is the difference between a relay and a contactor, and in what situation would you use either?

Below is a definition of both, these are not mine but what I found on the net, are they correct.

Relay
An electrically controlled device that opens and closes electrical contacts to effect the operation of other devices in the same or another circuit.

Contactor
A magnetically-operated device, for repeatedly establishing and interrupting an electrical power circuit. It is usually applied to devices controlling power above 5kW, whereas the term 'relay' is ordinarily employed below 5kW. The terms are often used interchangeably.

Jim