What is electricity?
I have wondered the meaning of that question for quite a while. I'll give an informal definition, what I use when I consider it.
First, there exists an electromotive force between all charges. This simply means that charges cause each other to move. Opposite charges attract and like charges repel. This is a fundamental force of nature, akin to gravity, but much, much more powerful. It can be helpful to think of the electromotive force as a type of gravity, except that it can both repel and attract.
Charges create an electric field, similar to how mass creates a gravitational field. The Earth, for example, creates a gravitational field that draws other mass (like the moon) to it. Likewise, a positive charge creates an electric field that can attract or repel other charges.
The gravitational field of the Earth does work; it applies a force over a distance. What this means in practical terms is that it takes energy to "fight" the field. If you jump straight up, you have to fight the gravitational field. If you fall down, you are being accelerated "for free". Potential energy is the energy something has due to its location in a field (with respect to some point). Anything higher than you has more potential energy because it can fall to your level "for free". If you want to reach the same potential energy level, you must do work.
Likewise, charges can have potential in an electric field. It takes work to move a negative charge away from a positive one - you are adding to its potential energy. Electric fields have two types of charges, so you can gain potential energy by moving two like charges closer or separating two opposite charges. The concept is the same as gravitational potential. The charges have a place that they would like to be.
Voltage
Voltage is the difference in potential between two points in space. Often, the Earth is assigned zero potential (called ground). You can think of voltage in terms of gravity as how far you lift a mass off the ground. The higher you lift the weight, the greater its potential energy with respect to the ground. Yes, the analogy works quite well, does it not?
Voltage varies with the amount of charge. If there are more charges "pulling" your charge, then lifting it away gives it a greater amount of potential. Lifting a barbell on earth is much easier than lifting the same barbell on Jupiter, because of its greater gravitational field. In essence, voltage is a measure of how eager the charges are to return to zero (ground) potential.
Moving Charges
If we place charges in an electric field, they will have a tendency to move. You may have seen diagrams such as ones in the figure. They show the forces acting on a positive charge.
Electrons are mobile charges - they can move from atom to atom. Protons are relatively static - they stay in the nucleus with the nuetrons. Thus, when an electric field is applied, electrons are the ones that move. However, the convention in use follows the flow of positive charge. Thus, when positive charge flows, it is really electrons flowing in the opposite direction. The location where the electrons are leaving is gaining positive charge.
Electrons under the influence of a field jump from atom to atom. However, this usually only occurs if there is a chain of atoms - the field pulls the electrons in one direction. The first electron is pulled off, and the next one jumps in, and the electrons are replaced. This is why circuits must be closed in order to work -> the supply of electrons must eventually be replenished.
Electricity
Electricity is the name we give to a series of moving charges. A Coulomb is a unit of charge (insert amount here). An Ampere is defined as a Coulomb of charge passing a location in one second. Thus, an Amp is (amount) electrons passing by per second.
Dissipating energy
Now, these electrons in high potentials must disspiate their power. Using the gravitiational example, a falling object converts potential energy to kinetic energy (energy from movement). When they finally hit the earth, they must convert their kinetic energy into something. Usually this comes out as sound, heat, and adding kinetic energy to other objects.
Electrons going from a high potential to ground (or a lower potential) must dissipate their energy. The most common way to do this is through heat.
Resistance
Think of resistance as a type of tax electrons must pay. Resistance is a property of the material conducting electrons. Good conductors are said to have low resistance. They have many free electrons, and part easily with them (have a low tax). They are usually shiny because of this (see light reference). Poor conductors have atoms that do not want to part with their electrons, and have a high tax. Thus, few, if any, electrons can be transmitted across the material.
As electrons pass through a highly resistive material, they expend much of their energy trying to move (paying the tax). Thus, they do not have much left to transfer to heat (or other useful sources). As electrons pass through a low resistance material, it takes little energy to move through it, and thus much of their energy is transferred to heat.
(explain arcing, creating a potential great enough to cause air to conduct... lightning..huge voltage difference)
Resistance determines the amount of electrons (amps) that can flow. The higher the resistance, the more tax, and the more difficult it is for electrons to flow. For low resistance, there is less friction, and electrons can easily flow.
Now, resistivity is a property of a material in general (iron, copper, lead, etc.). Resistance is the property of a specific piece of material. Resistance decreases with surface area (more electrons can flow at a time). Resistance increases with length - the longer the material, the more atoms the electrons must hop, and the more "tax" they must pay.
Movement of charge
Mother nature does not like to do work. She will minimize energy, and will do work when forced. Thus, when electrons have two paths to take, they will use the one that minimizes energy used (least tax), namely the path of least resistance. However, once a path has its maximum amount of amps passing, other paths will be used (even if they have a higher resistance).
Views of electricity
Hmm... alternate view of electricity: pulling charges from metal.
An electric field tries to move charges in a material. Materials with a high resistance "resist" this pull, and thus few charges move.
Thus, the field is everywhere, and certain materials allow certain amount of charges to flow. This is a property of the mat
