From Comment is free:
The Tor Project, originating from the US Naval Research Laboratory to assist defence and intelligence services in anonymous internet browsing, today comprises one of the most successfully and widely used anonymity networks. Maintained by a worldwide collective of supergeeks (or, rather, security and anonymity specialists), it uses several thousand servers around the world to disguise your internet identity and the destination of the website you are wishing to visit.
To make Tor unblockable in China and other countries, a new feature has recently been added. Instead of joining the Tor network directly, thereby revealing your intention, you first connect to a computer set up by your friends or colleagues, who then introduce you to the Tor network (a “virtual bridge”, they call it). Because the Chinese cannot know in advance who these friends of yours will be (technically speaking, their IP address), they cannot pre-empt by blacklisting. Once you do connect through the bridge to the Tor network, it is almost impossible for surveillance agencies to know that you are using Tor.
Read also Global Internet Freedom Consortium (GIFC) Offers China-Based Reporters Software to Break Through Internet Blockade via GFW Blog, and Chaos Computer Club offers help to victims of censorship in China from Chaos Computer Club e.V.