CNN - The government in China has built a patchwork system of controls that include software to root out offensive keywords and block blacklisted web sites. Blog entries are automatically rejected if they trigger a keyword filter.
Government censors, known as Net nannies, surf the web looking for pornography, subversive political content or other illegal material. The government also imprisons people who mail, post online, or access politically sensitive content within China.
China is among a handful of countries that have extensive filters for political sites. Iran, Myanmar, Syria, Tunisia and Vietnam also strictly block political content. Link.