It's much in the news that Google in China (google.cn) is blocking access to certain websites, at the request of the government. One story said those who wish to get uncensored information "have long ago figured out how to circumvent the censors. They use proxy servers, to give just one example."

What's a proxy server? How does it circumvent censors, and is it used for any other, more general purpose?

In this sense, it is a computer that stands in for another computer. Instead of connecting directly to the internet, and to evade detection, those computers in China are going through an anonymizing proxy server. All data that is transfered between computers is done so in chunks (called packets), and two pieces of key information in each packet is the IP (internet protocol) addresses of the originating computer and the destination computer. (These IP addresses are four 8-bit numbers, called octets, separated by periods, e.g., 192.168.0.1 is an IP address.) These numbers can be collected and stored on either the source or the destination computers, and any computers inbetween that the data packets travel through. The numbers can be associated by the ISP (internet service provider) to a customer. Google stores all the searches that come from a particular IP address. The president has recently asked for all of Googles IP address correlated searches to discover what folks in the States are looking for on the ineternet.

Proxy servers are also used by companies to regulate and track employee access of the internet through firewalls, for security and to make sure that internet access is not be abused. In computer jargon, a server is a piece of software running on a computer that offers a service: e.g., a mail server, web server, database server. The computer accessing the service of a server is called a client.