The Relay is a digital normally open switch that controls a relay capable of switching much higher voltages and currents than your normal Arduino boards. When set to HIGH, the LED will light up and the relay will close allowing current to flow. The peak voltage capability is 250V at 10 amps.
Please exercise great care when working with mains voltages— if in doubt contact a professional such as a licensed electrician for help.
The Raspberry Pi Relay Board gives your Pi the ability to control high voltage/high current devices, easily makes normal home appliances become intelligent devices.
The Grove - 4-Channel SPDT Relay has 2 single pole/double throw (SPDT) switches. It only requires low-voltage and low current signals to control the switches.