Electricity is a generic name given to a whole series of quantities and laws that we will shortly investigate. Every quantity we introduce shall have a letter associated to it which identifies it within the formulae and the circuits it is in. The most important physical quantity is the current (I) which is measured in Ampere, generated by the flow of electrons through a conductor. The latter, like any physical element, is composed of atoms, an atom being a particle with a nucleus which has a certain charge, conventionally defined "positive" and a certain number of electrons (with a negative charge) which orbit around the the nucleus (clearly things are in fact a lot more complicated, but this being a sound engineering course and not a Quantum Mechanics course, we feel justified in simplifying things so as to aid us in our dissertations, so long as we don't digress too far from reality). The electrons are kept in position by the nucleus whose opposite polarity attracts them. In fact two elements of opposite polarity attract each other, whilst two elements with the same polarity repulse each other. The force with which these two poles attract each other varies depending upon the kind of atom (in other words, depending upon the kind of material we are considering): with metals the attractive force is very weak, with insulating materials attractive force is far more difficult to overcome. As we shall see, this is the reason why metals are excellent conductors whilst insulators are not. Let's consider a bronze cable -an excellent conductor- to which we'll apply, at each end of the cable, respectively a negative and a positive charge.
Electrons belonging to the bronze atoms within the conductor, being of negative polarity, shall be attracted by the positive charge and repulsed by the negative charge. Because of the fact that the attractive force is very weak in conductor materials, the electrons are "stolen away" from the nucleus, thus generating a flow of charges (q). The measuring unit for the quantity of charge is the Coulomb (C). At this point we have enough information to state that a current is quantity of charge that travels through a conductor within a unit of time (1 second). Current is measured in Ampere [5 ]. Let's not better define the charges we have applied to the conductor. A local accumulation of positive or negative charges is called potential. By applying two different charges at the opposite far-ends of a conductor, a difference in potential is generated, which is called tension (V) and is measured in Volt. By applying a tension to the far-ends of a conductor, a flow of charge is generated, whose value depends upon the applied tension and the characteristics of the conductor.
When the current or the tension are constant in time, we call this continuous tension or current whilst when they vary in time we call this alternate tension or current. A classic example are normal house-hold electric sockets, in which we have an alternate current with sinusoidal rate with a frequency of 50 Hz and a width of 220 Volt.
[5 ] As with all physics formulae, pay attention to not confuse the greatness symbol (I for current) with the symbol for its quantity (A for Ampere which gives the amount of current). We'll say that a current I measures 5 Ampere, for example. By definition 1 Ampere is the current generated by a charge of 6.26x1018 electrons which passes through a conductor in 1 second. Translated into formulae: Q= I x t (Q=charge, measured in Coulombs, I=current, measured in Ampere, t=time, measured in seconds).
Related topics on Wikipedia
Generation of a current