[b]Voltage[/b] - the difference in charge between two points.
[b]Current (Amps)[/b] - the rate at which charge is flowing. (It is the amount of [b]coulombs[/b] (6.24*10^18) passing a point in one second)
[b]Resistance[/b] - a material's tendency to resist the flow of charge (current).
[h3]Alternating Current - AC[/h3]
Alternating Currents are currents which periodically change in direction.
The direction of the Current is changes between around 50 or 60 times a second, this is called the frequence (Hz).
It alternates between a negative peak and a positive peak so looks something like this:
Due to the fact that the charge changes, the Voltage changes as well, so the voltage is plotted on the [b]y[/b] axis of the graph, rather than the direction - and time is on the [b]x[/b] axis to represent the change.
The alternation of the current is measured using an instrument named and [b]oscilloscope[/b].
[h3]Direct Current - DC[/h3]
Direct Currents are much simpler, it is simply the flow of electrons from one terminal to another with no oscillations.
The above is an oscilloscope graph's view of a DC current.
According to classical conventions, the electrons went from the [b]negative terminal[/b] of a battery/cell, to the [b]positive[/b] one.
However in reality the current flows from the [b]positive terminal[/b] of a battery/cell, to the [b]negative[/b] one.
The classical convention is called [b]Conventional Current Flow[/b].
The real one is called the [b]Real Current Flow[/b].