The management regulations is approved by the State Council of China on December 11 1997 and promulgated by the Ministry of Public Security of China on December 30, 1997. This is a legal basis. In that regulations, a detailed definition of "harmful information" the specific meaning, publishing "harmful information" is forbidden. Then what is the "harmful information"?
According to the Section Five of the Act provides that no individual may use the Internet to create, replicate, retrieve, or spread the following kinds of information:
(1) Inciting to resist or breaking the Constitution or laws or the implementation of administrative regulations;
(2) Inciting to overthrow the government or the socialist system;
(3) Inciting division of the country, harming national unification;
(4) Inciting hatred or discrimination among nationalities or harming the unity of the nationalities;
(5) Making falsehoods or distorting the truth, spreading rumors, destroying the order of society;
(6) Promoting feudal superstitions, sexually suggestive material, gambling, violence, murder,
(7) Terrorism or inciting others to criminal activity; openly insulting other people or distorting the truth to slander people;
(8) Injuring the reputation of state organs;
(9) Other activities against the Constitution, laws or administrative regulations.
Evidently, these nine categories of information is harmful information. I think, in one word, any threat to social stability statements are "harmful information". Anyone want to publishing "harmful information" is illegal. This is the so-called "Internet censorship in China", If you knew about this, you will be able to understand those weird things in China's cyberspace.
Sources:William Long: The internet censership's law basis (Chinese)