An old adage about the Internet is that it "interprets censorship as damage and routes around it." The more you try to restrict access to something, the more ways people find to get to it.
Governments like those in Iran or mainland China place restrictions on the Internet with software, and individuals work their ways around those restrictions with more software. The end result is an arms race: here, a country blocks YouTube or Facebook; within days (or even hours), people inside and outside that country engineer ways to work around the block.
There's no one way to do this, but they all have a few things in common. They require some participation by people on the other side of the firewall, who can allow requests for non-blocked content to be used to deliver blocked content. How they do this varies, but that one technique lies at the heart of just about all efforts to circumvent censorship.
I'm going to look at several of the major software technologies used to perform that kind of circumvention. Some of them require nothing more than installing a simple software package; some are more convoluted. Each of them comes with risks and shortcomings, which in turn also must be worked around.
Tor (The Onion Router)
Tor is nominally used for the sake of anonymity, but also works as a circumvention tool, and its decentralized design makes it resilient to attacks. It started as a U.S. Naval Research Laboratory project but has since been developed by a 501(c)(3) nonprofit, and is open source software available for a variety of platforms. Human Rights Watch, Reporters without Borders, and the United States International Broadcasting Bureau (Voice of America) all advocate using Tor as a way to avoid compromising one's anonymity. With a little care, it can also be used to route around information blocking.
The concept behind Tor is simple enough. Out there are a whole slew of servers that are part of the worldwide Tor network. Connect to one as a proxy, and your Internet requests are routed at random through other servers in the Tor network. Requests between Tor nodes are encrypted. By the time the request emerges from Tor's network and is sent on to the server in question, its origins have been heavily obfuscated. If you want, it is possible to pick a specific entry and exit node, or even to forcibly exclude specific exit nodes.
The advantages ought to be clear. For one, there's no immediate way to tell where the connection is originating from, geographically: a request made in the United States could emerge from the Tor network somewhere in Poland. Another major feature of Tor is the hidden service protocol, which makes it possible to use the Tor cloud to anonymously publish a Web site or provide other network services, although only for people directly connected to the Tor network. Tor also works with just about any Internet application, since it works via the SOCKS proxy interface.