Web Anonymizers are special web sites that let you access other web sites while making it impossible for them to have any information about you (like knowing your IP address, planting or reading HTTP cookies or having any information about your location).
Many people use web anonymizing services to maximize their privacy or bypass web blocking applications (see our article: Web blocking software can be easily bypassed).
However, it seems that those services have flaws that allow web sites to bypass those anonymizers and retrieve information about your system.
The leading web anonymizers are simply web sites that proxies HTTP requests. The client computer accesses their web sites, and after typing the URL of the requested web site, the anonymizers will retrieve and display the HTML page using their server instead of the client machine. In this case, the remote server knows nothing about the client machine, only on the anonymizer server.