As it is most commonly used, a mirror site is a website that replicates an existing website, and is frequently used to reduce the load on and improve the availability of the original website. Mirror sites are quite useful when a site generates too much traffic for a single server or network to handle.
Some mirror sites are live mirrors, meaning that they are updated automatically when the original site is changed, using software to coordinate the synchronization of the two sites. Other mirror sites must be updated manually, and serve primarily as backups or working copies of the original.
Cached mirror sites allow for the archiving of older or unsupported content, permitting website operators to upgrade the primary site while maintaining access to outdated and unsupported content on a virtual copy of the primary website.
Mirror sites have also been used in a political context, as a means of opposing censorship by remotely replicating sites that have been banned, seized or otherwise disabled by governments. This approach has been used by sites ranging from Google, when it was banned in 2002 by the Chinese government, to more controversial sites like Wikileaks and many other commercial websites as well.