Having your Arduino control higher-power devices like lights, motors, pumps, doors, and many more is one of the most interesting and useful applications you may get involved with. But it can be a little difficult and possibly dangerous when power line voltages are being controlled.
This wiki section covers many of the considerations about having your little Arduino control large amounts of power using relays, Power FETs etc. http://arduino-info.wikispaces.com/ArduinoPower
I am still developing this and other sections of the site. All comments, critiques and suggestions are appreciated!
Regards, Terry King firstname.lastname@example.org