Start by considering a step down transformer that produces 120V. The secondary has two terminals, and 120V AC measured between these two terminals.
Now set up a second transformer, with its primary in parallel with the first and its secondary in series with the first. The secondary of the second has 120V between its terminals, but there is now 240V between the first terminal of the first transformer and the second terminal of the second transformer.
You could add yet another transformer if you wish (still same phase), and add yet another 120V, so that between any of the secondaries on a particular transformer you would have 120V, but you would have 360V at the 'ends' of your string of transformers.
A center tapped secondary is essentially two transformers with parallel primaries but secondaries in series.
-Jon
P.S. I started writing a long essay, I've included what I wrote below, but I think the above makes the essential point.
Okay, lets go in sequence:
1) Voltage. Voltage is a measure of the work needed to move a charge (say a single electron) from one point to another. The greater the voltage, the greater the work needed to 'push the charge up-hill', or conversely, the greater the work that can be obtained by letting the charge 'fall down hill'. Voltage is _always_ measured between two points, like height. It is meaningless to say 'that mountain is 1000 feet high', you need to say 'that mountain is 1000 feet above the surrounding terrain', or 'that mountain is 1000 feet above sea level'. Likewise with voltage; it is always 'that electrode is 100V positive relative to that other electrode'.
2) Alternating current. Alternating current is a system where, given two conductors supplying a load, if you measure the voltage between the conductors you will find that it is continuously and periodically varying, starting at 0V, going to positive maximum, then dropping back to 0V, then negative maximum, and then back to 0 again. This variation is happening continuously and (hopefully) smoothly. If you were to draw a graph of voltage versus time, it should be a sine wave.
4) More than two wires. With three or more conductors, you have several different voltages that you can measure. Since voltage always is measured between pairs of points, you can have as many different voltages as there are pairs of wires. For example in a three wire system, there are three different voltages that you can measure A to B, B to C, and A to C.
4) Polyphase alternating current. A system of multiple conductor power delivery where if you measure the voltage between any pair of wires you have alternating current, where if you were to graph voltage versus time you would get a sine curve, and where the sine curves for symmetric pairs of conductors are equally displaced in time. So, for example, in a three phase three wire system, with three possible voltages, all of the sine curves have the same amplitude and frequency, but are displaced from each other by 1/3 of a cycle (ideally!)
