The difference between alternating current (AC) and direct current (DC) is readily explained, yet most widely published "explanations" seem to muddle the facts and cause confusion. I hope that this short article may help to clarify matters a little.
A Little Etymology
Originally, alternating current was called "alternate" current and direct current was called "continuous" current.
In the presence of the existing diversity of opinion regarding the relative merits of the alternate and continuous current systems, great importance is attached to the question whether alternate currents can be successfully utilized in the operation of motors. The transformers, with their numerous advantages, have afforded us a relatively perfect system of distribution, and although, as in all branches of the art, many improvements are desirable, comparatively little remains to be done in this direction. The transmission of power, on the contrary, has been almost entirely confined to the use of continuous currents, and notwithstanding that many efforts have been made to utilize alternate currents for this purpose, they have, up to the present, at least as far as known, failed to give the result desired.
A new system of alternate current motors and transformers
N Tesla, American Institute of Electrical Engineers, 1888
I think that Nikola Tesla may have been using the term "alternate" current as a businessman's term rather than as a scientist's or engineer's term. After all, his system was "alternate" to the already established direct current electrical systems. The term "alternating" is more scientifically apt: the voltage polarity and the current direction really do reverse (alternate) regularly in AC supplies.
So, what is the difference between AC and DC ?
Haven't you worked it out yet? Surprise, surprise! Er, that should be: Supplies, supplies!
An electrical cable or indeed any current-carrying component of a system can conduct electricity along a prescribed path for the purpose of carrying a signal, or for carrying power, or sometimes both. Every electrical or electronic component that has been or ever will be built exhibits the properties of inductance and capacitance even if only to the minutest degree. These two properties introduce distortions into any and every electrical or electronic system.
DC is often described as if every part of an electrical circuit is either at a constant voltage or is at zero volts. This is wrong. The ever-present inductance and capacitance ensure that if there is any change in the applied voltage this will cause voltage and current changes in the load circuit which do not necessarily track linearly with the changes in input voltage. Under some conditions when an inductive load is switched in or out rapidly, the voltage at the switching contacts can reverse polarity. Yes, DC can exhibit reverse polarity! If you don't believe me, just check out Lord Kelvin on electrical oscillations. A Leyden jar is a device for storing static electricity. Benjamin Franklin connected a number of them in series to create a higher voltage, and called his device a battery. It was most definitely what we would today call a DC device. Lord Kelvin, a British scientist, was the first to observe the oscillating (reversing) nature of the discharge of electricity from a Leyden jar through an inductor. That was some time before Tesla's practical applications of electrical oscillations.
AC or DC - It's a supply-side thing.
Any explanation of AC versus DC which focuses on the load is certain to be wrong, wrong, wrong. It's a supply-side thing.
If the supply voltage is meant to reverse polarity, especially at a regular rate, then we are talking AC supply.
If the supply voltage is not meant to reverse its polarity then we are talking about DC supply.
It is quite wrong to try to explain AC or DC in terms of the load. The power source will supply electrical power with either constant polarity (DC) or reversing polarity (AC). It is far too late now to get away from the older terms, but if we had begun with CP for constant polarity and RP for reversing polarity then there might be less confusion. And maybe, just maybe, in a parallel universe there is a heavy metal band called RP/CP.