Relays are an oldie but a goodie. They allow you to switch high current/voltage loads using a digital output from something like the Arduino.
What would make a relay even better? How about controlling a relay over the internet from anywhere in the world? Yea!
This process is made really simple using the Teleduino sketch for your ethernet enabled Arduino. Haven't heard of Teleduino? That's cool, you may want to take a look at Arduino Control via a Web Service with Teleduino
to get yourself started.
This tutorial will guide you through the process of attaching a 5V relay to your Arduino, and provide some example Teleduino API calls to:
Define the pin mode of a pin
Turn on the relay
Turn off the relay
Toggle the relay
If you get stuck on any of the steps, or if you feel that there is a step that could be explained better, please leave a comment so that we can improve things.
Let's get started!