YouTube online again in China (for two days-see update)
As of March 27, YouTube was available again in China following a four-day layoff, reportedly over a video showing police brutality in Tibet that authorities insisted was fake. In recent years, China's web censors (colloquially known as the Great Firewall or the Net Nanny) have relaxed censorship policies on the English language web, but still obstruct many popular blogging services and nonprofit organizations (a list of notable targets can be found on Wikipedia).
Prominent news organizations such as the New York Times and CNN are available in China (regardless of content), as are Wikipedia and Blogger (both of which were blocked as recently as two years ago). Google caches are always blocked in China (disabling the "view as HTML" option for PDF files) and Google image searches can only be gleamed for a few pages before turning into pages of dead links.
The YouTube block did much to draw the ire of ordinary Chinese, who have grown accustomed to using the service. Chinese authorities usually do make any public acknowledgments or explanations for blocks that are imposed. Most of the actual blocking which takes place is done on the ISP level, where service providers are expected to know what websites are and are not permissable. Google and other search engines have come under criticism for censoring results on their Chinese-language home pages.
UPDATE: By March 30, YouTube was down once again across China, according to reports registered on Herdict.