Phase difference between two waves is the difference of the phases of the two waves.
y1 = A Sin (w1 t + A)
y2 = A Sin (w2 t + B)
the phase difference between two waves at time t is = [ (w1 - w2) t + (A- B) ]
For the same wave phase difference between two points in space is the difference in the argument for Sine function. It is equal to difference in the phase angle corresponding to the two points x = x1 and x = x2 at time t1.
y1 = A Sin (w t1 - kx1)
y2 = A Sin ( w t1 - kx2)
phase diff = k (x1 - x2)
If phase difference is zero, then both waves are exactly in same phase - identical in their wave nature. If the phase difference is 180 deg., then when one wave is at maximum, the other is at minimum. They are opposite.