It is important to wire a LED the right way around. Note how on a new led, the cathode (negitive) is shorter than the anode(positive), and the led has a flat spot on the cathode side. On most leds, the anvil is the cathode side, but this is not always true, as with the LED in the photograph.




Batteries too are an important thing to get the right way around. This is a 9 volt battery. The + side is positive.



Resisitors can be attached either way.  The coloured stripes on a resistor indicate its resistance.