they have a current limiting resistor built into them (which most do not have). An LED will draw all the current it can until it melts/blows...so a resistor is used in series with the LED to limit the current to around 20 - 30 milliamps. It does not really matter if you place the resistor on the + (anode) or - (cathode) side of the LED. Typically a resistor around 330 ohms to 1K ohms will work for most 12 VDC LED applications...determine the resistor value based on how bright you want the LED to shine (lower resistance the brighter the LED).