Tor, The Network Browser
The internet is an incredibly powerful tool designed to transfer information. These days you can find just about anything with a simple google search. Unfortunately, not everyone in the world has access to an unfiltered internet like we do in the US. Many organizations (such as high schools) and countries (such as China) censor parts of the internet in the interest of controlling information. Essentially, they are restricting access to specific sites, such as facebook, because they have no control over the content.
In order to understand how these organizations censor we first have to understand the underlying structure of the internet. Essentially, the internet can be thought of as a giant network of servers (nodes) connected to each other with links. In reality, these links are cables that crisscross the entire world, and even go under the oceans to connect continents together. Whenever we plug our computer into the internet and try to access a site, we connect to a local router which then sends a request for information through the network to the server hosting the website. The website (say Facebook) receives the request, and then sends information back through the network to your computer. Each node in the network required to connect you to the site is able see the destination site (say Facebook) along with the location of the computer who requested the information (you).
The methodology used to block sites is to block the link between your computer and the servers that host that site. For instance, in order to block Facebook you only need to block all connections to the Facebook site. As long as the organization/country controls a node they can stop the connection to Facebook by refusing to continue the chain and returning their own message to you.
Tor manages to circumvent censorship by connecting you to the “tor network.” Essentially, instead of requesting information directly from the intended website (facebook) you request information from the tor network. The tor network consists of a bunch of servers agreed to essentially become a middleman in information transfer. Each server is only told the next server to connect to, and the final server is told the intended website (facebook). Then each sever sends information (which is encrypted of course) back through the network to the first server, which sends the information back to the user. This means that no server in the entire network knows both the intended site and who is trying to access the site. Additionally the people trying to censor you just see that you connected to a random server, they do not even know it is part of the tor network (many servers in the tor network are also used for other purposes like hosting other sites). This allows the user to access blocked sites, and do so privately.
https://www.torproject.org/about/overview.html.en