The Peoples Republic of China is roughly the size of the USA, and has the worlds largest population with 1.4 billion people. It is so large it borders 14 different countries.
Hong Kong, Macau and Taiwan, although officially considered part of China, have different immigration and visa requirements and their own laws and currencies.
Hong Kong and Macau are considered Special Administrative Regions of China. Taiwan is different, with its own separate Government which was set up by defeated nationalists after the civil war in 1949.
Taiwan considers itself completely independent of China however they are not recognised by China or the United Nations, with China considering it one of their territories.