A visa is a travel document issued by authorized government agencies to foreign citizens applying to enter, exit from or transit through the host country's territory based on the host country's laws and regulations. In accordance with international law and practice, any sovereign state is entitled to determine whether or not to allow a foreign citizen to enter or exit from its territory, and whether to issue a visa, decline a visa application or to cancel an issued visa in accordance with its national laws.
Chinese embassies, consulates and other institutions stationed abroad entrusted by the Ministry of Foreign Affairs of China are visa-issuing authorities responsible for issuance of entry visas to foreigner citizens abroad, and are entitled to determine the type, number of entries, validity and duration of each stay of a visa to be issued, or to decline a visa application or revoke an issued visa, in accordance with Chinese laws and regulations. According to Chinese law, a foreign citizen may be refused entry into China even if he/she has a valid Chinese visa.
Foreign citizens must obtain a visa before entry into China, with the exception of visa-free entry based on relevant agreements or regulations. Foreign citizens that fall into categories of Article 22 of the Exit and Entry Law of the People's Republic of China are qualified to apply for port visas from visa-issuing authorities entrusted by the Ministry of Public Security at a port of entry approved to issue port visas by the State Council.
For visit to Hong Kong SAR or Macao SAR, citizens from some countries/territories are exempted from visas for short term visit. Otherwise an entry visa must be obtained as required by the Government of Hong Kong SAR or Macao SAR. Visa applications must be made separately if the traveler wishes visit both Hong Kong SAR/Macao SAR and mainland China.
Chinese visas fall into four types: diplomatic visa, courtesy visa, service visa and ordinary visa. Ordinary visas are further divided into 12 sub-types or 16 categories.
Visa Validity ("Enter Before") means that the visa is valid, or can be used for entry into China from the date of issue to the "Enter Before" date indicated on the visa (Beijing Time). If a visa has unused entries, the bearer can enter China before 24:00 (Beijing Time) on the expiration date.
"Entries" refers to the number of times the bearer is permitted to enter China during the validity of a visa. A visa becomes invalid if there are no entries left, or there are entries left but the visa validity expires. If a visa becomes invalid, its bearer must apply for a new visa before entering China. Traveling with an invalid visa to China will result in refusal of entry.
"Duration of Each Stay" refers to the maximum number of days the visa bearer is permitted to stay in China each time, which is calculated from the date of entry into China.
Residence Permit: A holder of category D, J1, Q1, S1, X1 or Z visa must apply for a residence permit at the local public security authorities within 30 days of entry into China unless the Duration of Each Stay on the visa is marked as 30 days. Members of foreign diplomatic or consular missions in China must apply for a residence permit at the Ministry of Foreign Affairs or local foreign affairs departments within 30 days of entry into China.
http://www.china-embassy.org/eng/visas/zgqz/
中华人民共和国驻美利坚合众国大使馆