Phase velocity, Group velocity

Theoretician

Would someone be kind enough to please give physical meanings to these two terms? I have never fully understood their meaning and difference (although I know how to express them mathematically).

If, say, I have a Gaussian-shaped signal in the frequency domain that I am sending through a medium (such as a crystal) in the direction z say and I want to specify where the maximum intensity is as a function of t (it is at z=0 when t=0), am I right in saying that I would use:

t= z/(group velocity)

where the group velocity is obviously evaluated at the frequency of largest intensity.

Any help on this fairly basic matter would be much appreciated.

Doc Al

This discussion might help: http://www.physicsforums.com/showthread.php?t=96835

Theoretician

Thank you, I shall have a look but I actually think that I have a good enough understanding now having read a few relevant sections in the Feynman Lectures (sorry, this is what I should have done first).