Complex sentence: The main difference between these two groups is the behaviour of the phase velocity when the frequency approaches zero, where the first group has a zero-valued phase velocity, and the second group has a finite, nonzero phase velocity.
Split into simpler sentences: The main difference between these two groups is the behaviour of the phase velocity when the frequency approaches zero. Whereas the first group has a zero-valued phase velocity, the second group has a finite, nonzero phase velocity.