The flow of electric current in a conductive material can be compared to the flow of water through a hosepipe. A motive force is required to cause current to flow in a conductor and the motive force is voltage. The higher the voltage the higher the current flow, just like opening the tap further increases the water pressure in the hosepipe causing more water to flow.
Some materials make better conductors than others due to their atomic structure and their place in the Periodic Table. At the centre of each atom is a nucleus consisting of particles known as protons and neutrons. Neutrons are electrically neutral and simply “glue” the nucleus together stopping the protons which are positively charged from repelling each other. Orbiting around the nucleus are shells of electrons arranged at varying distances. The further away they are from the nucleus the weaker the bond is to the nucleus, making it relatively easy to eject the furthest electrons from their orbits making them free to join other atoms.
An atom contains the same number of protons as it does electrons. The attraction or ejection of electrons from an atom leaves it with a net positive or negative charge and the atom then becomes known as an Ion. It is the movement of these “free” electrons between atoms that gives rise to electricity. A material that allows the movement of “free” electrons is known as a conductor, and one which doesn't is an insulator. Electrical current is simply the movement of free electrons in a conductive material.
As previously mentioned a motive force is required to induce current flow in a conductor. Placing two dissimilar metals in a chemical solution (electrolyte) causes a reaction to take place. Electrons will travel from one of the metal plates through the electrolyte and collect on the other plate causing it to have an excess of electrons giving it a negative charge. The difference between the plates is referred to as potential difference. This is the basis of a simple battery.
For an electrical current to flow there are two conditions that must always be met. Firstly there has to be a means of motive force (voltage) to force current to flow, and secondly there must be a continuous circuit for the current to flow round. Placing a piece of wire between the plates creates a continuous circuit that will allow the electrons from the negative plate to flow through the wire back to the positive plate due to attraction between unlike charges. This process will continue until the chemical action of the battery is depleted and there is no potential difference between the plates. Placing a low voltage bulb into the circuit gives a visual reference to show that there is a current flowing between the plates. This demonstrates the drift of free electrons that we know as electricity.
It is worth noting that the flow of electrons is always from negative to positive through the conductor, whereas conventional current flow shown in a circuit diagram is from positive to negative.