The components required for Inputs and Outputs will depend on what kind of devices you are connecting. AC or DC? 12V, 24V, 120V, 240V. If you want to do any analog you will probably have to change from 0-5V on the Arduino to somehow have 0-5V, 0-10V, -5 - +5V, -10 - +10V, 2 - 20mA.
It all depends on what you want to connect to.
One option is to look at Opto-22 devices.