Anonymous Proxy

"Client - server" is the most common model of exchanging information in the Internet. A client sends a request (request for files) and a server sends a reply (required files). For full support and understanding between a client and a server the client sends additional information about itself: a version and a name of an operating system, configuration of a browser and etc. This information can be necessary for the server in order to know which web-page should be given to the client as some browsers may not be compatible with the web-page's design or technologies used. However, as long as web-pages do not usually depend on browsers, it makes sense to hide this information from the web-server.

What your browser transmits to a web-server:

Name and version of the operating system
Name and version of the browser
browser configuration (display resolution, color depth, plug-ins, javascript support, adobe, etc)
IP-address of the client
Other information

The most important part of such information is information about the IP-address. Using your IP it is possible to know about you the following:
a country where you are from
a city
your provider
your physical address

Anonymity

Anonymity at work in Internet is determined by what environment variables are "hidden" from a web-server.

If a proxy server is not used, then environment variables look in the following way:

REMOTE_ADDR = your IP
HTTP_VIA = not determined
HTTP_X_FORWARDED_FOR = not determined

According to how environment variables is "hidden" by proxy servers, there are several types of proxies

1. Transparent Proxies

They do not hide information about your IP address:

REMOTE_ADDR = proxy IP
HTTP_VIA = proxy IP
HTTP_X_FORWARDED_FOR = your IP

The function of such proxy servers is not the improvement of your anonymity in Internet. Their purpose is information cashing, organization of joint access to Internet of several computers, etc.

2. Anonymous Proxies

All proxy servers, that hide a client's IP address in any way are called anonymous proxies

3. Simple Anonymous Proxies

These proxy servers do not hide a fact that a proxy is used, however they replace your IP with its own:
REMOTE_ADDR = proxy IP
HTTP_VIA = proxy IP
HTTP_X_FORWARDED_FOR = proxy IP

These proxies are the most widespread among other anonymous proxy servers.

4. Distorting Proxies

As well as simple anonymous proxy servers these proxies do not hide the fact that a proxy server is used. However a clients IP address (your IP address) is replaced with another IP:

REMOTE_ADDR = proxy IP
HTTP_VIA = proxy IP
HTTP_X_FORWARDED_FOR = random IP address

5. High Anonymity Proxies

These proxy servers are also called "high anonymity proxy". In contrast to other types of anonymity proxy servers they hide a fact of using a proxy:

REMOTE_ADDR = proxy IP
HTTP_VIA = not determined
HTTP_X_FORWARDED_FOR = not determined

That means that values of variables are the same as if proxy is not used, with the exception of one very important thing which is a proxy IP is used instead of your IP address.