Anyone who has ever had anything to do with China and the Chinese will immediately be able to tell you that there are 56 ethnic groups in China. Of course, in most countries of the world if you asked the average person how many ethnic groups the country holds you might get confused looks and different answers, however in China everyone knows the exact answer: there are 56 ethnic groups.
The reason for this is of course simple: the Chinese state officially recognizes 56 ethnic groups. A big thing is made of this in much of the government's official propaganda, with slogans like "the 56 peoples are one family" and similar. Whenever the National People's Congress convenes, you will see photos in the papers of representatives from minorities in their traditional costumes, which are actually only worn in daily life in remote rural areas.
In fact, the 56 peoples are by no means equal in size. As most of you will know, the Han form by far the biggest group, comprising over 90% of the population of the People's Republic of China. The remaining 55 ethnic groups are the minority peoples, or 少数民族 (shaosu minzu) in Chinese.
The Han are in many ways identifiable with China as such. The Chinese language is their language (in fact one of the names for it is hanyu or "the Han language" in Chinese), and what is usually considered to be Chinese culture is basically their culture. However, the official line is that the 56 ethnic groups of China are all Chinese, and that being Chinese does not depend on your race. Most of the minority groups conserve at least some of their own customs and their own language. Their languages usually belong to completely different families from the Chinese language.
The 55 minorities can be quite large by European standards. The largest minority are the Zhuang, with around 16 million people. The better known Tibetans are more than five million. At the last count, 18 of the minorities passed the million mark. The smallest minority are the Lhoba of Tibet, who only number a few thousand (however there are far more of them over the border in India apparently). Unsuprisingly, some of the classifications are rather dubious, with some small groups being lumped together with other ethnic groups which they don't necessarily feel part of, and other small ethnic groups still unrecognized (there is even a Chinese term for these unrecognized peoples: 未识别民族, or wei shibie minzu).
To understand why the issue of relations with the minorities is crucial in China, it is necessary to realize one fact: although the minorities constitute less than 10% of China's population, the areas where they are the majority cover over half of China's territory. This is because many of them are concentrated in the scarcely populated but vast Western territories like Inner Mongolia, Xinjiang, Qinghai, Tibet etc..
The Eastern part of China consitutes the heartland of the Han. Made fertile by the Yellow River and the Yangtze rivers, it is extremely heavily populated, containing about 90% of China's people. The arid and less hospitable Western and Northern reaches, as well as the South-West, are the homeland of most of the minorities. Although most of these areas have been under Chinese control for centuries, the Chinese cultural influence has always mixed with other influences, and the population is a patchwork of different ethnic groups and religions. Particularly over the last centuries, the Western and Southern regions where most of the non-Han groups live have received large numbers of Han settlers from other parts of China, and in many cities the Han are now a majority.
The history of relations between the Han and the surrounding peoples are complicated.The craddle of Han Chinese civilization was the North China plain, but the Han gradually expanded southward to satisfy their growing population and demand for land. In so doing, they came into contact with a variety of ethnic groups, some of which fled, others stayed and were assimilated by the Han, while others retained their distinct identity. At the same time China was often conquered and subjugated by nomadic peoples from the north, most recently by the Mongols and by the Manchu. However all the invaders quickly assimilated the far more advanced Han civilization.The Han used to look down upon the "barbarians" who surrounded their world, and their culture was indeed far more sophisticated by most standards.
The minorities have assimilated Chinese culture to varying degrees. The Manchu, the people from North-Eastern China who took control of the whole country in the seventeenth century and created the infamous Qing dynasty, are now almost totally assimilated, and their language is spoken only by a few e
lderly people. Another group, the Hui (who the women in the photo on the left belong to), are basically just Hans who practice the Muslim religion, and are therefore given a separate classification. However some minorities, like the Tibetans and the Uyghurs, remain very different from the Han in their lifestyle, language and sense of identity. Resentment against the Han still exists within a few minorities, most notably in the case of the last two peoples mentioned. In the general chaos of nineteenth century China there were various rebellions against Han rule. For instance, the Dungan and Panthay rebellions by the Hui or Chinese Muslims led to a decimation of China's Muslims, with a few million being killed, and many others fleeing to Russia. Nowadays violence still occasionally flares up in Tibet and Xinjiang, something which I'm sure you've all heard about. However in the rest of the country there is generally no violence between different groups.
Every Chinese citizen's ethnic group (or minzu in Chinese) is registered on their identity card. Being a Han or belonging to a minority makes a real difference to a Chinese citizen. Due to a kind of affirmative action policy, the minorities are exempt from the one child policy, and they are given preferential access to university places and government jobs. Although Communist party members are officially not supposed to practice a religion, this rule is relaxed for minorities, since otherwise it would be very difficult to allow people from some of the minorities to become members.
The provinces with the highest prevalence of ethnic minorities are also given the statues of "autonomous regions", with special provisions for the protection of the local language and culture. Tibet, Xinjiang, Ningxia, Yunnan, Guanxi all enjoy this status. Although the governor of these provinces has to be from the local minority, the party secretary who is the real power broker is currently a Han in every single one of them. The languages of the minorities enjoy official status, and any Chinese banknote has the value written in Tibetan, Uyghur, Mongolian and Zhuang as well as in Chinese. This is in contrast to the situation with the different dialects of Chinese spoken in different provinces, which are not recognized and are even discouraged by the state. However, as one might imagine, anyone from a minority who wants to get anywhere in modern China has to learn standard Mandarin Chinese.
In the Western world, where since the French revolution the ideal has been that all the citizens of a country are equal regardless of their ethnic group or religion, the idea of having your ethnic group recorded on your identity card might seem quite strange. However, the system currently followed in China was actually influenced by the practices of the Soviet Union, which followed a similar policy towards minorities. A similar system is also followed in Vietnam, which recognizes 54 groups rather than 56, and in many other countries around the world (in many Middle Eastern countries you are classified by religion rather than by ethnic group).