For example,
a path difference of l/4
corresponds to phase difference of
rad, and
a phase difference of p rad corresponds to a path
difference of
.

We can rewrite the equation describing wave 2 as:

Thus, we can say that wave 2 is
shifted with respect to wave 1 by a distance of
f/k
(See Math note1).
This shift is called the path difference between the two
waves. Sometimes, the path difference is called the path shift
and the phase difference is called the phase shift.
A phase difference of
f corresponds to a path
difference of f/k=(l/2p)f.

Suppose you have two waves described by:

The phase of wave 1 is kx-wt and the phase of
wave 2 is kx-wt+f.
The phase difference between the two waves is (kx-wt+f)-(kx-wt)=
f.