High voltages and Arduino

I want to control light, fan and motors in the greenhouse. All of these use 220 VAC, so I need to get some info about (safe) high voltage control. If you are planning to control high voltages (220 VAC 20 A), make sure that you know what you are doing - its lethal. Some instructions can be found here. An example of parts that you will need can be found at Electrokit.

A relay, an electrically operated switch. Source: http://www.electrokit.com/tinkerkit-rela-230v.51853