Proxy servers, Free proxy list, Anonymous and transparent proxy servers
What is an anonymous proxy server?
An anonymous proxy server (sometimes called a web proxy) generally attempts to anonymize web surfing. There are different varieties of anonymizers. One of the more common variations is the open proxy. Because they are typically difficult to track, open proxies are especially useful to those seeking online anonymity, from political dissidents to computer criminals. Some users are merely interested in anonymity for added security, hiding their identities from potentially malicious websites for instance, or on principle, to facilitate constitutional human rights of freedom of speech, for instance. The server receives requests from the anonymizing proxy server, and thus does not receive information about the end user's address. However, the requests are not anonymous to the anonymizing proxy server, and so a degree of trust is present between that server and the user. Many of them are funded through a continued advertising link to the user.
Access control: Some proxy servers implement a logon requirement. In large organizations, authorized users must log on to gain access to the web. The organization can thereby track usage to individuals.
Some anonymizing proxy servers may forward data packets with header lines such as HTTP_VIA, HTTP_X_FORWARDED_FOR, or HTTP_FORWARDED, which may reveal the IP address of the client. Other anonymizing proxy servers, known as elite or high anonymity proxies, only include the REMOTE_ADDR header with the IP address of the proxy server, making it appear that the proxy server is the client. A website could still suspect a proxy is being used if the client sends packets which include a cookie from a previous visit that did not use the high anonymity proxy server. Clearing cookies, and possibly the cache, would solve this problem.
Source: Wikipedia (Full article)