Now let's see LED's. The main specification of LED are its current rating=20mA, typical cut in voltage=2V,life time=2lakh hours,max. voltage is around 4.5V. There are different color LED's depending on the semi conducting material

LED has two leads- cathode and anode. They are identified by the length of the lead. Cathode lead is of lesser length. But I have seen some LED's with manufacturing defect having cathode lead longer. So in order to identify the cathode of the LED see the figure below. In that you can see that cathode is of broader filament.I got some white LED's of cathode of small filament. So this convention can be right or wrong. Check LED in both ways to see that LED is good.