How does electricty work?  Despite tons of examples in nature - such as lightning, electric eels, and static electricity - it took thousands of years for humans to start writing out defensible answers to that question.

Consider how water flows from a higher location to a lower location. We can put this flow to work through a water wheel.  When water falls it collects energy from gravity, which we then can convert into rotation of a wheel.  Similarly, we now know that electricity is the flow of electrons from a high-energy state to a low-energy state.  In between, we can make the electrons do work, lighting up bulbs and turning motor shafts.  The analogy is so useful, that the flow of electrons is called an electrical current.  Ben Franklin observed that electricity was trying to move from one charged state to another, so he named one positive and the other negative.  Franklin and his contemporaries decided that current flowed from the positive terminal to the negative terminal.  Unfortunately, electrons actually travel in the opposite direction!  This is more than a little frustrating.  You now understand this comic.

Consider this circuit but don't build this.  You could burn out an LED.  A battery has a positive terminal and negative terminal.  When we wire one up to a light bulb, as shown above, electrons flow across and light the filament as they travel.  They then go back to the battery.  This round-trip is known as a circuit.  Note that the bulb must consume energy from the electrons, so if you were to remove the battery or use one with insufficient charge, it would fail to power the bulb.

The light-emitting diodes (LEDs) we will be using in this class work differently, and electricity can only flow across them in one direction.  Also, it would be a bad idea to wire up your LEDs like the bulb shown above - it could burn out!    Include a resistor, a material that inconveniences electrons and reduces their flow.  It looks like this:

It would be a mistake to say that resistors slow down electrons, but rather the carbon inside these resistors convert some of the energy in the electrons into heat.  

Put a resistor in the circuit, somewhere between the bulb and either end of the battery, so that the electrons will be forced to pass through it in their path around the circuit.  If at first it doesn't light up, turn it around and try the other direction.  That's better!

Note that you can place the resistor either before or after the LED in the electron's path, with the same results.  So electricity isn't completely like water.  You need to evaluate the whole path to predict how the electrical current will flow.