'Fans' are supporters. Sports teams and celebrities have fans. If you say 'I'm a fan of ..' , it just means that you like that thing or that person. For example 'I'm not a fan of tofu' mean that I don't like it much. Being a 'fan' is about personal preference. 'Buffs' are both interested and knowledgeable about a topic. For example, a 'cinema buff' is a fan of the cinema, but he/she also knows a lot about the history of the film industry, for example. Being a 'buff' is about knowledge.

谢谢hunter的问题，更谢谢Su.Ki.的回答，我一直以为fan就是“非常喜欢一个人或一件事的人，因为汉语的”迷“是这个意思。不知道光喜欢还不能算，还要支持。谢谢了。