The main principle of any proxy server is providing a mediation between web-sources and individual users. Frankly speaking proxy-server is an additional computer (more intelligent, powerful and capacious than your home PC) that is used as a third agent for any reason. It is made for numerous purposes which are described in the following article.
1. The first proxy server’s aim is to keep users’ anonimity during surfing the Web. Some web-providers say that anonimity is the only purpose of a proxyserver but they are partly-right: proxy keeps anonimity only for the highest web-provider, but the local network (or proxy-server) administrator still gets all the logs of particular user’s requests. If there is a need this data can be used to find the criminals, to block any user or ban some web-sources, to avoid terrorism or misdemeanour.
2. Another aim of proxy server is the opposite: it is used to provide the access of local network users to the Web. In the very beginning of networks’ development there was no “Global network” because program machines were connected with a limited number of other machines to transfer data, increase capacity etc. Even later during the gradual development of the Internet the importance of local networks remained. They still provide faster access to specific data than to web servers that may be located far away. Proxy servers in this case are used as a kind of "hub" for data transfering. F.e. providers’ proxy server is still sometimes used to storage the most popular files (and even website "mirrors" sometimes) for local users. They could download these files or access the necessary site at a relatively high speed comparing it with Internet. Today almost every user has a broadband access to the Internet, but proxy servers are still used as the cache servers.
3. Proxy server can also be used with the aims that are opposite to described ones: web administrator can configure the ban on the use of certain sites or websources. This is true for some companies, as well as educational institutions, and even whole regions and countries - in any situation when there is a need to restrict access to websites, files, or other data. F.e. in China and Democratic People's Republic of Korea (and many other countries) proxies are used to ban "anticonstitutional" websites.
This are major goals of proxy servers that may be varied or combined, depending on specific requirements.