It is an SLA-12VDC-SL-A 4Pin 30A Power Relay. The relay is switched with the help of a small signal from a microcontroller like an Arduino. This relay can be used to drive 30A devices.
Applications:
Used for power switching in Electrical Heater.
Used for power switching in ventilators.
Used for power switching in air conditioning, Refrigerating.
It's definitely worth buying. It's a genuine product, and I've bought two sets, both of which are working great. If you're looking to use it in DIY projects, go for it!