Switching a 125VAC light bulb with a general purpose Relay Relay 5VDC by BARRAGAN http://barraganstudio.com

Relays enable you to control one circuit from another circuit with no electronic connection between the two circuits for example to control a device (switching it ON or OFF) from the Wiring harware. A Relay is an electric switch capable of switching AC devices by applying a small current at it, making possible to control high AC voltages from the Wiring hardware. It is always very useful to consult the datasheet of the optocoupler you're using for specific needs.

int relayPin = 8;

void setup()
{
  pinMode(relayPin, OUTPUT);  // sets the digital pin as output
}

void loop()
{
  digitalWrite(relayPin, HIGH);   // switch ON the triac
  delay(1000);                    // waits for a second
  digitalWrite(relayPin, LOW);    // switch OFF the triac
  delay(1000);                    // waits for a second
}