I understand that Uf should be Vf. But on the LEDs I get they all say Uf
http://snootlab.com/lang-en/leds-10-mm/57-led-10mm-green-diffused.html
http://snootlab.com/lang-en/leds-5-mm/51-led-5-mm-green-20-8x.html
The important thing is that I now understand that these values are forward voltage and what they are used for.
For an output pin of 5V with a LED of Vf 2V and an lf of 20mA I would use a resistor of 150 ohms
(5 -2) / 0.02 = 150
For an output pin of 3.3V with a LED of Vf 3V and an lf of 20mA I would use a resistor of 15 ohms
(3.3 - 3) / 0.02 = 15
As regards the circuit, if I've understood correctly and following the examples I've seen in the books, I run the power from the output pin on the Arduino (3.3V or 5V) to the anode wire of the LED, a wire from the cathode wire of the LED to the resistor (using the calculation above) and a wire from the other end of the reistor to the digital or analog output pin of the Arduino.
I think I am, very slowly but surely, getting there.
I've been following the examples in the books, and blindly plugging in the right components in the right holes, without really understanding why.
Thank you all so much for your explanations.