Electric charge
Charge is a physical property of object that exerts a force on other charged objects, similar in how the earth's mass generates gravity that pulls us towards it.
There are two types of electric charge, named positive an negative. Two subatomic particles are responsible for the charge of objects, electrons with a negative charge, and protons with a positive charge; neutrons are not charged particles, so they have no effect in the object's overall charge.
As I mentioned earlier, electrically charged particles exert a force on other charged particles, and the polarity of the charge determines where the force is directed: particles with the same charge will exert a force that push them away from each other, particles with different (opposite) charges will exert a force that pulls them together.
It takes energy to push two opposite charged particles apart, but the force that pushes them together again can be used to do useful work, same with particles with the same charge pushing them away from each other.
Now, imagine that there are two charged particles some distance apart in a frozen moment in time. If we start the clock, the force that pulls or pushed them toward equilibrium (as close as possible for opposite charges and as far as possible for same charges) will do a certain amount of work and use a certain amount of energy, the same work and energy that took to place them where they were before we started the clock.
Since we know that the force will do some work and use energy, we say that it has a potential: the ability to perform work that has not yet been expended. We can only define the potential energy that a charged particle has relative to another charged particle, if there's no other charge then no force is generated, so when we speak of potential it is most often in terms of a potential difference between two points.
Circuit variables - Water system analogy
When defining the variables in a electronic circuit, it is easy to picture it as a closed water system with a pump and a length of pipe filled with stones.
Three main variables exist in any electronic circuit:
Voltage: voltage is the potential difference created inside a battery or power source. Voltage can be though of as the water pressure that the pump generates.
Current: electric current is the movement of electrons in a circuit. By convention (that is, everyone agrees that it is like that), current is said to flow from positive to negative, or in terms of potential difference, from a higher (more positive) to lower voltage (more negative). Think of current as the amount of water that passes through the pipe.
Resistance: It is a material's opposition to the flow of electric charge, similar to friction and any moving object. The stones in the pipe represent the resistance of the circuit, the smaller the stones and the more of them, the harder it is for water to pass through.
Ohm's Law
A man by the name of Georg Simon Ohm discovered that there's a simple relation between voltage, current and resistance, given by the formula I = V/R, where I denotes current Intensity in Amperes (unit of current flow), V represents Voltage (a unit of potential difference) and R represents Resistance in Ohms (unit of resistance).
This same formula can be rewritten in terms of each other to generate an equation for each electric characteristic in a circuit:
I = V/R
V = I*R
R = V/I
An easy way to remember the ohm law is to draw a triangle with 'V' at the top and 'I' and 'R' at the bottom, when you need to find any of the variables, you cover it with your finger and it will give you the formula (same row means multiply, one over the other means divide).
