I don't know if this is correct or not, but since China used to be a collection of many, many different nations before unification, each of these nations had its own ethnic group. As time went by one of the Chinese ethnic groups (Han) began to grow and expand, and many other ethnic groups assimilated with the Han. Later on the government classified Chinese people into 56 ethnic groups, of which Han is about 90% of the mainland Chinese population. The rest are referred to as minority ethnic groups because they did not assimilate with Han and still retain their own languages/dialects and other cultural aspects. Again, I don't know if this is correct or not. I moved to North America as a kid, so didn't learn any Chinese history growing up.