A proxy server is simply a program that relays data from one system to another. There are a number of free proxy servers available designed to offer the users some type of "anonominity" or access to "restricted" websites.For example, if your IP address was 1.1.1.1 and you connected to the internet through a proxy server with an IP address of 2.2.2.2 everyone would see you as connected to the internet with IP 2.2.2.2 not 1.1.1.1. Or at least that is the idea.
There are a whole host of applications for proxy servers and issues associated with them. Although many are used to privacy or anonominity this is not necessarily a feature or benefit.
For example, many proxy services are designed to pass on the HTTP_X_FORWARDED_FOR value, which would tell any server you connect to through the proxy service that you are using a proxy server at 2.2.2.2, but your real IP is 1.1.1.1. This is useful in many business applications where the objective of the proxy service is not privacy, but something else.
How do you know if that proxy server you are using for privacy passes this variable or not?
Since a proxy server relays data through the proxy server system, it is possible for data to be logged and/or modified. If you want to have real fun, modify the code in a proxy server to change all the letters "i" to "a" and see someone try to use it. Not useful, but very illustrative of the power the operator of a proxy server have.
When you enter your login and password, it will be relayed through the proxy server, but will it be logged too? How will you ever know.
Now the proxy server can provide a real benefit when you want to access blocked websites say from China or your school. Of course any decent blocking program would easily decode the proxy packet and block the sites direct or through a proxy server.
Now there are some real dangers of proxy servers and that is with respect to crime. I have seem some enticing sales pitches for a public proxy server you can setup for people to access blocked sites.
Basically the pitch goes like this... install our program and watch the money roll in as users use your proxy on your server to access blocked sites.
That also means that when someone uses your proxy to commit credit card fraud or hack into something, the victim will see the IP address of your server not the bad guy!
open proxy servers are another bad idea. This might be installed by spyware or they might be installed by mistake. They will turn your computer into a proxy server and you might not even know it!
There are many people that scan the internet for open proxy servers about publish lists of open proxy server IPs.