Utility poles
Regarding three-phase power:The electrical outlets in your home have two power-carrying prongs. The third prong is just for safety (it connects to the ground), and should never carry power unless something is going horribly wrong. In that case, the power should hopefully prefer to reach the ground through that third wire than through your human body.Anyhow, the voltage in those two power-carrying wires is constantly switching directions. At one moment, the left wire will be +170 volts relative to the right wire, and then they will slowly switch places over the next 1/120 of a second so the right wire is at +170 volts. They continue trading places, completing a cycle every 1/60 of a second.At some point in this cycle, the voltage difference between the two wires will be zero, which means that no power will be flowing at that moment (power = voltage * current). On the average, the wires will deliver the same amount of power as-if they had a constant 120 volts between them, which is why people say that electricity in the USA runs at 120VAC. The peak-to-peak voltage is 170V, but the "RMS" average voltage is 1/√2 of that, or 120V.Anyhow, those brief moments of time where no power is flowing are a problem for a power company, who would like to deliver energy in a continuous stream. So, they build their power system with three wires. Each wire reaches its maximum voltage 1/180 of a second after the previous wire. Since the voltages on each wire are sine waves, when one wire is at 0, the other wires are at +√3/2 and -√3/2 of their maximum voltages. Therefore, there is no point where the power stops. In fact, due to a mathematical quirk, the power delivery is actually constant, even though the voltages on the three wires are constantly changing.Turing three-phase voltage into two phase voltage is pretty easy. Just pick any two of the three wires and hook them into the home. The difference of any two sine waves is just another sine wave at the same frequency, so you automatically have single-phase power. In practice, the power company will try to balance the load between the three pairs of wires by sending different pairs into different houses or even neighborhoods.Edit: As several people have pointed out, this isn't quite how it works in real life. See the comments below for details about the hot vs. neutral wire and how both 240V and 120V are available in the home at the same time.