Under the "one country, two systems" approach, Hong Kong is supposed to be independent when it comes to making
internal decisions, but when it comes to foreign affairs and defense, it is under Beijing.
In fact though, many important decisions have to pass consultation with the Office of Hong Kong and Macau Affairs, of which Zhang is director. The Office of Hong Kong and Macau Affairs is like a sub-ministry set up to manage relations with these two areas because they do not fall under Chinese domestic control, and reports directly to the State Council, China's cabinet. In the case of Hong Kong and Macau, they are special administrative regions (SARs) of the People's Republic.
In addition to handling Hong Kong affairs, Zhang is the third ranking member of the Politburo Standing Committee, being only two steps away from Chinese president Xi Jinping. He is also chairman of the National People's Congress, and deputy head of the National Security Commission, which is chaired by President Xi Jinping. (The National Security Commission is a new body modeled on the US National Security Council, and started by President Xi Jinping two years ago.)
This means that Zhang is really, really important in the Chinese party and government hierarchy. Basically, he is the third most important person who runs China.
CY Leung, who is Hong Kong's chief executive, likely reports to Zhang Dejiang on a daily basis, especially following the demonstrations in Hong Kong. Another way of looking at it is that CY Leung is Zhang Dejiang's front man in Hong Kong. CY Leung cannot make any important decisions about the protests without first consulting Zhang, who would then report to Chinese president Xi Jinping.
Zhang Dejiang is originally from Liaoning in northeastern China, and went to university in North Korea. That's right, North Korea.
After finishing university, he was made deputy party secretary of Jilin, which borders North Korea. (The deputy party secretary would be like lieutenant governor in a US state.) His first major challenge was dealing with the issue of refugees from North Korea into China.
Later, he was made party secretary of Zhejiang province (like a US governor). Zhejiang has a very lively private sector; Alibaba, which just went public in the US, is from Zhejiang province.
In 2002, he was made party secretary for Guangdong, which borders Hong Kong. As party secretary in Guangdong, he worked with the Hong Kong government to make travel and transport between Guangdong and Hong Kong more convenient for business people. He also worked with Hong Kong's leading business people on their factories and investments in Guangdong, and providing support for their investments. This way, he built a working relationship with many of the tycoons who play a leading role in Hong Kong's economy.
He also met with many of the leaders of Hong Kong's democracy movement.
In July 2011, he dealt with the cleanup following the Wenzhou train disaster. In March 2012, following the sacking of Chongqing party secretary Bo Xilai (who was later sentenced to life in prison), he was appointed Chongqing party secretary.
After successfully cleaning up in Chongqing, he was appointed to several senior positions by Chinese president Xi Jinping.
Chinese president Xi Jinping heavily relies on two people: Vice Premier Wang Qishan, who has managed the party anti-corruption campaign which has taken down so many Communist party officials and Zhang Dejiang, for everything else. You might say that Wang Qishan handles party affairs and party discipline, while Zhang Dejiang handles non-party affairs. Both of these men have proven themselves by successfully cleaning up messes; this is their value to President Xi Jinping.
The party anti-corruption campaign has been especially tough: there are reports that there have been six assassination attempts against President Xi Jinping by party insiders under investigation, or who fear investigation. The travel itinerary for Vice Premier Wang Qishan is a state secret, just for this reason. In speeches to party members, President Xi Jinping has repeatedly stated that it is impossible for the party to be defeated by outside enemies; the only way it can collapse is because of a breakdown of internal party discipline. He has also said that he is prepared to sacrifice his own life in re-establishing party discipline.
If you want to understand how Beijing is going to react to the democracy demonstrations in Hong Kong, Zhang Dejiang is the man to watch.
Read this for more on Zhang Dejiang