Single phase has 2 energized wires, which creates a "phase" between those leads. Three phase has 3 energized wires which creates a phase between each pair of wires, so 3 phases. Maybe
@Ron can elaborate on the difference between 120v and 240v nomenclature, since one has 2 hots and a ground, and 1 has 3 hots and a ground. Wire is generally sized based on how much amperage draw the circuit or equipment has. Equipment generally requires a certain amount of watts to run. For a single phase circuit, Watts=volts*amps So a 4800 watt load pulls 40 amps at 120v, 20 amps at 240v, or 10 amps at 480v. Also, 3 phase reduces amp draw by about 43% compared to single phase. In industrial applications, where you're running thousands of feet of wire or pulling thousands of amps, it can be significant cost and complexity savings to use smaller wire and higher voltage.
In the residential world, it just means that you can buy whatever equipment you want. Some of us fools who like equipment have to spend hundreds or thousands of dollars on phase converters and transformers to get 440-480v 3 phase power. Because of this, the same equipment on 120v or 220-240v single phase is more valuable to the average consumer because everyone has that power available at their house. Now that I have a good phase converter, I don't mind 240v 3 phase stuff, but still stay away from 480v 3 phase stuff because I don't want to buy a transformer to convert from 240v to 480v. When I was building my shop, I asked Duke Power about running 3 phase to my house. They said I would have to pay for all the lines and new poles from the main highway, down the secondary road, down my tertiary road, and down my driveway, which is probably about 2 miles. I didn't even bother with a quote. If you ever go to sell your place, it will add some small value to the right person because you have 440v 3phase power.
