What kind of info are you looking for?
A relay outlet is fairly straightforward- you use a low voltage from one device to control the 110VAC power to another via a mechanical or solid state relay.
IMO, an arduino would be much simpler than raspberry pi
You would begin by choosing an input pin and output pin for each appliance you want to control. Input pins connect to a switch, output pins to your relay(s).
You then write a simple ‘sketch’ for your arduino, instructing it to turn on the output pin when the input pin switch is closed and off when open.
Upload the sketch to your arduino and connect everything up
Control 4 outlets with this:
https://www.adafruit.com/product/2935
Or you could just buy switches and run extension cords...
edit: fixed bad link