The rotor, the stator, the stator cover (and the other cover I mentioned?), and the fixed advance point cam bwaller mentioned are the magneto parts.
Don't confuse this mag with the Fairbanks-Morse mags used with the ARD mag drives; it's the same machine, but in a much simpler package.
The way a magneto works is to generate a pulse of current when the permanent magnet moves across the poles of the stator. When the current is at it's greatest, the points open, and you get a spark. This is why the rotor has to be phased properly. It's the same type of system used with dirt-bikes, lawn-mowers, chainsaws, etc. Most of this systems don't have any provision for headlights or battery charging, but some do. Since the Yosh was advertised as a racing unit, it probably doesn't.
If you look at any stock SOHC stator, you will see the inside is cut into a bunch of segments, with a bunch of coils. Each of the segments is a pole.