Using relays are common way of switching high power loads with electronics. If you take any microcontroller or any other digital IC, you will see that their output current on single pin is very limited – varies around 20mA. Same situation is with voltage. Digital pin output voltage is limited to IS supply voltage like 3.3v or 5V. Usually we need to switch loads that draw significantly higher currents and are powered from higher voltage supply. And there you have several options for switching loads. One and most oldest method is using mechanical relays. They are still very popular way of switching power electronics. One biggest disadvantage of using mechanical relay is that it has moving parts with all rising problems. These are: slow switching time; relatively high control current; noisy; may produce sparks on switching; sensitive to environment factors like vibration, humidity; To overcome those problems there is more modern solution – a Solid State Relay (SSR). Instead of switching loads mechanically, SSR does this with help of electronics.