LEDs

Most LEDs work pretty much the same way. Standard package is the T1-3/4. LEDs want 5-20 mA of current (If) running through them. Current runs through the LED from anode to cathode. Cathode is marked by the short lead and/or a notch or flat on the case. Always place a resistor in series. When active, LEDs have a forward voltage drop (Vf). Here's some Vf/If data for an ordinary LED connected to a 5 V supply through a series resistor. The 330 ohm configuration is recommended.

Series Resistor Vf If
100 ohm 2.2 V 27 mA
150 ohm 2.0 V 20 mA
330 ohm 1.9 V 9 mA

You can run two LEDs straight off a single Arduino pin. Wire the LEDs in series and add a series resistance. Two 330 ohm resistors wired in parallel work nicely. To drive more LEDs off a single Arduino pin, wire in parallel and turn on and off via a transistor. For more than 4 LEDs, use a separate battery supply.