Quote Originally Posted by CVMichael View Post
Something to add to that: Every single device (regardless what it is) it has a resistance, including the LED...
So, in your example the voltage goes down because of the resistance of the LED is in serries with your 220 ohm resistor.
In fact, the LED will have two 'resistances' one forward (small) and one backward (large). The 'D' in LED means 'Diode'; which is a semi-conductor that, as the name implies, will pass current in one direction only. (up to a point)

You'll note that in these simple experiments, if you connect the battery the wrong way round, the LED will not light up at all. Also, if you put too much voltage the wrong way round you can destroy the device (it often lets the smoke out, which is not a good thing for most electronic components)