Imagine a circular track full of ballbearings or marbles or something. All packed in tight, so if you push on anyone of them, they all move around the track.
Now imagine that over a section of that track is a waterwheel-type thing; a wheel with paddles or cogs spaced so as to come down between balls as the wheel turns, and give them a push around the track. DC current is that wheel; it goes around in one direction and pushes the balls (charge) around the track in one direction constantly.
Now imagine that instead of that waterheel type thing, we have a pin or axis rotating about the vertical axis above that section of the track, with a paddle poking out to one side. The pin is in the track, so that as it turns, the paddle pushes the ring of balls first one way, then the other, constantly changing direction. This is AC current. At any point along the track, at any moment, there will be balls coming toward that point or moving away from that point, depending on which way along the track you are looking.
or for the nonanalogical...
Let me first define direct-current, or DC, voltage sources. A direct-current source has a positive end and a negative end, i.e. a battery, and when a wire is attached to both ends, (called a closed circuit,) electrons flow from the negative end to the positive end.
For an alternating-current, or AC, voltage source, it works the same way except that the positive end and negative end keep trading places periodically. That is, at first the positive end is at the "top" and the negative end at the "bottom", the current flows from the bottom to the top, then it alternates and the "top" is now negative with the "bottom" positive and the current flows from the top to the bottom. Then it alternates again, and again, and again...
The time it takes to switch the positive and negative "sides", or terminals, we call the Period of the AC current, from this we can get the Frequency of the AC current, which is 1/Period.