famous people是著名的人

如：The actions and words of famous people can have a great influence because many people may follow them.

great people的的意思是指伟大的人或民族

如：

The Chinese people are a great people. 中国人民是伟大的民族。

famous 侧重知名度，即广为人知，而great侧重名望，包含受人尊重的意思