4-channel relay control board with optical isolation.
Relays are the Hulks (tm -DC Comics) of the electronics world. Often dumb and simple, but DANG they can control lots of power. These are ideal in situations where you need to comfortably control AC or DC power levels. Transistors and FETs can do the same job, but often not with the convenience and reliability of a good old relay.
This particular model features optically-isolated inputs, meaning the 5V TTL control signal is not exposed to the potentially dangerously high power levels being managed in the relay. It's safe; isolated; secure! As long as your signal can drive 15~20mA to power the opto-isolator, you are fine!