Magnetic Contactor is generally a switching device, such as a Relay. Unlike general-purpose relays, contactors are designed to be directly connected to high-current load devices. Relays tend to be of lower capacity and able to switching a current rating bellow than 15 amperes, where as a magnetic contactor able to switching a device more that a few kilowatts. Specially it is designed with features to control and suppress the arc produced when interrupting heavy motor currents.
***It should be noted that though they are similar in design, magnetic contactors are not circuit breakers. They do not sever the connection between appliance, and power source during a short circuit.