Hi
channels with variable response to different frequency have fading. for example in multipath environments. if channel response yary fast, it is fast fading. if not, it is slow fading.
diversity means sending same data or symbols or informations from different way(in different time or by different frequency or with different antenna or...) to improve performance of system like BER.
I suggest u reading 1 good book for your answer, because it is not easy to explain this topic in short answer.