In a low power circuit or a output from a Microprocessor is very low. It is sufficent for a LED to glow but to drive a high load you will need a Relay (Electromagnet Switch), and to give proper voltage or current to a relay you will need a relay driver. Many times one transistor with a resistance is enough to make a Relay Driver. In this type of circuit Transistor is used as current amplifier and Relay does two things (a) they isolate current(flow of electron) this is important because high load appliances runs at different voltage(potential difference) thus Relay protects your sensitive electronics parts. (b) Relay is a electromagnetic switch. It is a type of machinical Switch which is pulled by a electro-magnet so its resistance is very low and thus it can control large power appliances.
Now days In market Relay Driver Module is available which is generally combination of relay and a transistor. In many modules LED’s is also placed to indicate status of a Relay Switch. In Market Relay module can purchased by telling how many Channel Relay is Required and driver voltage.
Advantage: You wiil be able to control various appliances, and other equipments with large current. It can be controlled directly by Microcontroller
Here is Picture of some common Relay Driver Module available in the market. Electronics Students uses it with Arduino and other microprocessor Projects etc