I'm not an electrician, but, as I understand it, homes get 240V from the power company, but it gets split to 120V for regular outlets. Basically, all electrical circuits have three conductors. One is always ground. For the other two, on 120V circuits, one is "hot" and carries 120V and the other is "neutral" and carries no voltage. On a 220V circuit, one is the same 120V as the "hot" on the regular outlets, and the other is also 120V running out of phase, so that the voltage difference between the two is 240V.

In reality, it's actually just a single 240V single-phase circuit, and the "neutral" is a center tap.

In more commercial settings, the power company provides three-phase power and there's about 240V between any two of the three phases. (I think it's actually 207V.)

There seems to be a lot of confusion over 110/220 vs 120/240. I'm not sure which is correct, to be honest.