delta(lambda) / (lambda)  =  v / c

The Doppler equation reveals the motion of an object, either
away or toward the observer.  Large motions produce large shifts.
The numerator on the left hand side is the difference between the observed
wavelength and the rest wavelength; if that value is negative, then the
motion is toward the observer; if positive, the motion is away from the
observer.  The denominator is the rest wavelength of the observed light.
The velocity of that motion is given as the numerator on the right
hand side; the denominator there is the speed of light.

When the velocity v approaches the speed of light, we must take special
relativity into account.  In that case, we use the "relativistic Doppler
equation":
                                  1 + v/c
  delta(lambda) / (lambda) = sqrt(-------) - 1
                                  1 - v/c

This version is important when computing, for example, distances to faraway
quasars using the velocity they are moving away from us due to the expansion
of the universe.  In that case, we use the letter "z" to denote the 
ratio delta(lambda) / (lambda), and call that ratio "redshift."