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.