If you put batteries for instance into a flashlight it lights up. But that will only happen if you turn on the switch. This is called: closing the circuit.
So, that is a circuit. Current flows from plus to minus.
The components of that image are arranged in a circle. Circuit in German is Stromkreis which means current circle. This illustrates pretty well, what current is doing in a circuit.
The batteries in a circuit are called current source. There are other current sources like power adaptor, generator or solar cells.
Current and voltage? What’s that?
Well, that is a very good question and not so easy to explain. Let’s compare electricity with a water pipe. The water in the pipe is under pressure. The more pressure in the pipe, the more powerful water squirts out of the faucet. If there wouldn’t be any pressure, no water would flow. The amount of water is like the electrical current, the pressure is the voltage.
Of course, there is no water in a socket. But there are little loaded parts inside. They are called electrons. The higher the voltage, the more electrons want to move. The movement of the electrons is the current.
Warning: Don’t ever connect something to a wall socket, what is not made for! The voltage of wall sockets are really dangerous!
So, now we want to build our own circuit, right? We will do that with the 5V which are provided by the Arduino board.