Relays and Contactors are electromagnetic switches. The only difference is that relays are usually used for low voltage applications while contactors are used for higher voltage application. Contactors usually have Overload protection.
A Relay is similar to a Auxiliary contactor,which is used in generally control circuit,specially in PLC for as a Digital input( feedback) or as a Digital out put to Switch on or off a electrical Contactor. we can also used a relay in place of a auxiliary contactor. relays are always used in a low current carring circuits. it comes with a rating from 5–20 Amps. while a power contactor generally comes with from 9 Amps to 600 Amps..
Difference between relays and contactors
Contactors and Relays perform the same task of switching a circuit. If you see from the application point of view, you would have seen contactors placed in control panels of industrial motors or other heavy loads. Whereas, relays are used in low voltage applications such as switching a LED or tube-light or even actuating a circuit breaker
Both are electromagnetic switches and operate under similar principles. The difference comes if we see from the application perspective. Contactors are used for high voltage switching purposes whereas relays are used for low voltage switching.