With this simple circuit you can control mains voltage devices like lamps, TVs etc with the 5V output of an Arduino.
Make sure to put a flyback diode at the relay (a diode that is placed in parallel with the relay, but in reverse polarity)
To protect the transistor against voltage spikes when the relay's coil is deenergizing.
Also make sure to put a resistor at the base of the transistor, so the output of the Arduino to limit the flow of current, which may not exceed 20mA at an Arduino! (Its absolute max rating is 40mA, but it seems like everything over 20mA shortens its lifespan and everything over 40mA fries it.)
