The two legs of a single phase 240vac circuit, like the one found in most houses, are NOT out of phase with each other. They are in fact exactly IN phase with each other, meaning that there is only ONE phase. If the two legs were 180deg OUT of phase (technically two phases), measuring the voltage between the two legs would show 0v. This occurs because as the AC waveform cycles, the two voltages would be exactly opposing at all times (Leg 1 +120v, Leg 2 -120v), making the resultant voltage 0. However, because they are phased Exactly the same, measuring the voltage shows double the voltage(Leg 1 +120v, Leg 2 +120v), as the two voltages are added together.
Three phase legs are 120deg out of phase, phase to phase. Normally adding three legs together would add all the voltage together (360vac). However, the phase differential causes some of the voltage to be lost to some of the voltage being cancelled by the other legs drawing it back. The line to line voltage is calculated by multiplying the line to ground voltage by the square root of three. Eg. 120 x sqrt(3) = 207.84, or 208vac 3ph.
The phase shift is often created at the generator, but can be created by the use of capacitors to delay the waveform, as in a static converter.
-Cody