The appliance that plugs in to it combines the two 110/120v currents internally to get the 220/240v. 3-wire outlets have 110/120v coming from the black and the red/white, and one ground.

I have a dedicated 110v outlet (on it's own circuit) a few feet from where I'm going to place the 220v outlet so I'm not sure if I'll run 12-2 or 12-3 but at least now I understand the difference ...