Hong Kong is a strange type of place. While it has been settled for the thousands of years that all of China has been populated, it is only a part of mainland China in a few important ways. It does not even have the same government as China does, electing its own Chief Executive (the head of state) and being autonomous is almost every possible area. Simply put, Hong Kong is a unique city that has taken over its island and rises higher than any other city in the world. Combining the best of the east and the west, there really is no other city on Earth like this lovely gem of a place. But while Hong Kong is still technically a part of China, it is only so in two important ways (which we will get to in a moment).
China is a different kind of country. Ever since 1997, Hong Kong has been back under the sovereignty of China, after Britain gave it up. Prior to that, Hong Kong had been a territory of the British Empire for well over a century. So naturally, while most of the people there are Chinese through and through, trying to muscle them under the systems that most of China lives beneath would most likely result in a civil war. After all, people hate to change, and the western influence of Great Britain.
Hong Kong is what is known as a Special Administrative Region, or SAR. While SARs are uncommon, they are the wave of the future as China begins to sweep countries like Taiwan back under its wing. In matters of national defense and international diplomacy, Hong Kong is China. But in every other way (including their courts, legislation and economy), Hong Kong acts just like another country in and of itself. While this might sound strange, it is a way to legally reconcile two influences that are too equal to have either side win out.