What is Internet censorship?

Internet censorship is control or suppression of the publishing or accessing of information on the Internet. The legal issues are similar to offline censorship.

One difference is that national borders are more permeable online: residents of a country that bans certain information can find it on websites hosted outside the country. Conversely, attempts by one government to prevent its citizens from seeing certain material can have the effect of restricting foreigners, because the government may take action against Internet sites anywhere in the world, if they host objectionable material.

Barring total control on Internet-connected computers, such as in North Korea, total censorship of information on the Internet is very difficult (or impossible) to achieve due to the underlying distributed technology of the Internet.

Source: http://en.wikipedia.org/wiki/Internet_censorship