Internet in China is heavily regulated, and the infamous "Great Firewall of China" is often cited in media as a prime example. Through this firewall, sites we take for granted are blocked or limited; famously, mentions of Tiananmen Square are censored, with users reportedly bypassing filters by referring to the incident as 'May 35'.
New legislation has entered pls that could impact Chinese internet use further, according to reports from state media. Before coming into effect, it will be subjected to revisions and discussion among the National People's Congress Standing Committee. Officially meant to “enhance protection of personal info online” and to “safeguard public interests," the rulings have been a cause of some concern due to internet registration with your real name. Some clauses exist allowing usage of pseudonyms, though it remains a controversial topic.
Posts deemed illegal can be deleted, even if you're not aware you are breaking any laws. Outlawed groups like the Falun Gong may find it difficult to communicate as a result of these changes. Sites like Facebook (which seems to be unavailable in China) likely will not have content removed, though China's own Twitteresque social network, Sina Weibo, most likely will remove illegal posts.