Where would you like to start? One thing to remember, is the majority of the Arduino I/O's are +5/0V logic, and the Raspberry Pi's GPIO pins are +3.3/0V logic. this means feeding +5V logic to the I/O pins of the Pi, can fry it. (the UNO R3, I believe is also 3.3V logic, But a little more forgiving to +5V signals, where as the Pi is not.) There are some sample circuits using NFETs, to transition the signal levels, but the raw circuit (NFET's and isolation resistors) make it impractical, BUT... I have seen recently, Adafruit is marketing a Level changer board (single chip, requires feed from the +3.3V of the Pi, and +5V from the Arduino.) You could use signals from the Pi, out to an arduino to handle mobility or higher voltage signals, and reduce the link between the two down to a I2C Clk/Dat wire pair, then use the Pi to send/receive signals from the net, or networked to a local computer to control them (as well as add video/sound.).