To start with, all matter is composed of atoms, which are composed of some basic particles of electric charges.  The smallest and lightest are called electrons and are referred to as negative charges.  The positive particle is known as the proton. As far as is known, all matter is made up of these particles.  Some of the characteristics are:  like charges repel each other and unlike charges attract.  This is similar to magnets, where like poles repel each other and unlike poles attract. A small experiment you could perform to see this would be to take a metal rod and attach 2 leaves of tin foil to one end and hang it in a glass jar.  If you would take a glass or plastic rod and rub it with some fur to charge it up, then touch it to the metal rod, the 2 foil leaves will spread apart.  This is due to the fact that that the static charge built up on the rod charges both leaves of the foil and they will physically move apart.

Atoms are composed of a nucleus containing protons that are 1840 times heavier than electrons.  Around the nucleus are an equal number of electrons flying around the nucleus like planets orbit the sun in our solar system.  How these electrons are arranged determines the stability of the atom.  The electrons are in different orbits, but the electrons in the outermost orbit are the ones that we will be observing.  For example the hydrogen atom has 1 proton and 1 electron spinning around it making it the lightest atom.  Typically the first ring is stable with 2 electrons.  The pull on the electron by the proton is offset by the centrifugal force of the electron flying around the proton keeping it in orbit.  Since it has one electron and 1 proton, its atomic weight is 1 and the elements go to uranium having an atomic weight of 92 with 92 protons and 92 electrons. Typically the electrons in the outer ring needs to number eight for stability.  

Lets look at a couple of atoms that are common in the electrical world.  The carbon atom has 6 protons in its nucleus, 2 electrons in the first ring and 4 in the second. The copper atom has 29 protons and 2 electrons in the first ring, 8 in the second, 18 in the third and 1 in the forth. Atoms that do not have 8 electrons in the outside orbit are not stable electrically. The farther from 8, the less stable. In the above examples the outer orbit of copper having 1 electron is less stable than the carbon atom with 4 electrons in the outer orbit.  This is a very complex subject and I have picked a couple of atoms and just tried to illustrate what basically happens.  If you desire any more information on this subject, get a good Physics book and it will explain it a lot better than I can in this small space.  Keep in mind that the sketch of atoms, below, appears like the orbits are all on the same flat plane. In reality electrons are like tennis balls and their orbits, which look like wire grids, make up the "shell" of the atom.  As another example, think of our solar system, and picture the sun as the nucleus of the atom and the planets as the electrons.

Now we will look at why these things are significant.  If you take a copper wire made of  "a gazillion" atoms and put a positive charge at one end this charge will attract one of the unstable electrons from the outer orbit of one of the atoms. This will leave this atom with one more proton than electron, so it will be unbalanced by 1 positive charge. T his atom will attract an electron from the outer ring of an adjacent atom.  The missing electron gets replaced by one from the outer orbit of an adjacent atom.  This activity continues from atom to atom creating an electric current.  The more positive the charge at the end of the wire the more electrons are moved.  The carbon atom with 4 electrons in its outer ring or valence is more stable than copper with only 1 electron.  If the same positive charge is applied to a carbon rod, since it is more stable fewer electrons will be attracted and the flow of electrons will be significantly less.  Therefore copper is a much better "CONDUCTOR" of electricity than carbon.  Also the thicker the material the more atoms there to give and receiver electrons, so the greater capacity for electron flow.

An atom that has 8 electrons in its outer ring or valence is stable and the electrons will not leave the orbit in response to a positive charge. This material is called an "INSULATOR". (These materials are also called DIELECTRICS.)

When an electric current is flowing in a conductor another curious thing occurs. The flow of electrons creates a magnetic field around the conductor.  Conversely another unique thing happens.  If you move a magnetic field across a conductor it causes the electrons to flow the same as applying a charge at the end of the conductor does.  What is happening is we are converting mechanical energy to electrical energy.  As we get into this deeper we will see how these factors are adapted to motors, generators, solenoids, transformers, ignition coils, voltage regulators etc.

THERE IS ONE VERY CONFUSING ITEM:  ELECTRON FLOW IS FROM NEGATIVE TO POSITIVE.  IN GENERAL PRACTICE CURRENT FLOW IS SAID TO BE FROM POSITIVE TO NEGATIVE.  In normal discussion people usually think of current flowing from positive to negative, and as we go forward, we will probably use that terminology. I think it is important that we have an understanding about atomically what an electric current is. 

There is one other law of physics we must remember and that is: "Energy can neither be created or destroyed, but may be transformed."  So as we journey forward, remember that electron flow may be transformed to mechanical energy, heat, etc. and these items may be transformed into electron flow.