The government of China has set up a system of internet censorship, intending to block internet users within Mainland China from accessing material deemed undesirable, such as foreign news sites, sites with dissident political content, many Hong Kong and Taiwanese websites, and pornography. However, some people in mainland China are buying software for them to access Google, Facebook,etc. This software is generally not very expensive. They use a version of Virtual Private Network or VPN to access those sites.
One part of the block is to remove some websites from search results on search engines. These search engines include both the local version of international search engines (e.g. Google.cn) as well as domestic ones (e.g. Baidu, Sohu). In general, regardless of whether a term is sensitive or not, many well known websites are removed from the search result, such as all porn sites, some western news websites like BBC and Voice of America, and a few sites in Hong Kong and Taiwan. In addition, access to a handful of US-based universities are blocked, as these websites often contain discussions regarding issues deemed politically sensitive by the Communist Party of China.
Some words are sensitive. Attempting to search for such a term may result in the turning on of the "safe search" feature, and limiting the result pages in China. However, the general internet traffic filter may interrupt a HTTP connection between the browser and the server if it detects intensive sensitive words in plaintext, as it does with other protocols, such as the Post Office Protocol, and any sequence connection to the server is also denied. This filter affects self-censored search engines, since their censorship is filtering websites, not keywords. This system is described in greater detail at Internet censorship in mainland China.
Any sequence containing the term is also blocked. For example, since 法輪 / 法轮 (falun, or "dharmachakra") is blocked, so are 法轮功 (Falun Gong) and 轉法輪 / 转法轮 ("Turning dharma chakra"). Also, only the Chinese terms are blocked, while the English terms are freely searchable (unless specified otherwise).
This list is not intended to be exhaustive. Most were verified for Simplified Chinese searches for the Baidu search engine. It is known that trying from different locations inside and outside China, on different search engines, and at different times can yield different results.
Because Google has decided to re-direct its Google.cn domain to Google.com.hk, censorship by Google is far less relevant.
"Tank Man" [in English] leads only to a terse warning about the questionable legality of the material one may be seeking in Google.cn. However, a search for "王维林," ostensibly Tank Man's real name, in the same search engine shows the Chinese-language Wikipedia's article on Tank Man as the first result.
not blocked: 王若水 (Wang Ruoshui) and 孫文廣 / 孙文广 (Sun Wenguang)