This wave has a period of p, noted above. Also notice that the start of the wave's period is at 0.
This is the same wave as the first, but its phase has been shifted. Notice that the period starts at the wave's highest point (1).
So what's the point? It just so happens that we have shifted this wave by one quarter of the wave's full period. We can shift it another quarter, if we wanted to, so the original wave would be shifted by half it's period. And we could do it one more time, so that it would be shifted three quarters of it's original period.
This means we have 4 separate waves. So why not let each wave stand for some binary value? Since there are 4, we can let each wave signify 2 bits (00, 01, 10, 11):
|Bit value||Amount of shift|
This technique of letting each shift of a wave represent some bit value is phase shift keying. But the real key is to shift each wave relative to the wave that came before it. Above is an example. Please note that I just randomly chose binary vales for each wave, and that the values shown are not correct!. The correct pattern should be: 00 00 10 00 10 00.