Hong Kong is not part of China. I don't care what the government says.
In Hong Kong, they speak another language (Cantonese not Mandarin). They use their Hong Kong Dollars instead of Chinese Yuan. You need a visa to enter China, but not Hong Kong. Flights from Hong Kong to mainland China land at the international terminal, and you have to then clear customs and immigration. You can even buy copies of 'Mao - the unknown story' in Hong Kong (this book is very critical of Chairman Mao, and is definitely banned in China).
Anyway, I don't have much else to say about Hong Kong, as I never actually saw any of it. I had a 12-hour layover there (on my way from Vancouver to Chengdu), but it was not possible to the airport.