The tribe

What is a tribe? Even in Japanese, I've found it hard to explain. I would say in Japanese "A tribe is a tribe and nothing else!" But I have to explain. Umm...in my impression, a tribe is a small group of people who live in a village and have lots of physical and mental similarities in common. The Japanese and Korean are the same race. But there are a lot of differences in its culture and way of thinking and so on. So we need the word "tribe" in order to distinguish one group of people from others. But I feel something strange to define the Japanese as one of the tribes probably because as the Japan's population gets larger, the less I feel we have in common.