The forward bias resistance of a stand alone LED is low. The forward voltage drop across them is between 1.8 and 3.3 volts depending on colour. As the voltage drop is fairly constant over a wide current range a higher applied voltage would rapidly increase it, so a series resistance is added as previously mentioned to limit same and avoid burn out. For the sake of the discussion a complete working LED as below has to be considered - sorry it's come out so big
They may also have a conventional diode within for reverse polarity protection or even a tiny bridge rectifier allowing them to be connected in a polarity insensitive manner. I know G4 fitting low voltage DC LED's as used on boats and caravans have such, probably many others including mains ones of course.
