A mirror site is a copy of a website or set of files hosted at a remote location. There are a number of reasons to establish a mirror site. You may be familiar with the idea of a mirror site, especially if you have ever downloaded software and been asked to pick from a list of download locations. Each location in the list was a mirror site.
There are several ways in which a mirror site can work. Most commonly, a mirror is a static copy of the original site, almost like a snapshot, requiring the owner to update the mirror frequently if he or she wants to keep the content current. It is also possible to establish a live mirror, which stays current with changes on the original site. Mirrors can copy entire websites, or they can serve as file archives, depending on the use that the mirror site is being put to.
One common reason to establish a mirror site is to cope with a sudden influx of traffic which would otherwise overload the server. By offering visitors a mirror site, or several, the site owner can keep the site running while ensuring that people get to see it. This can be useful when a site goes down because of a server problem or influx of traffic. Mirror sites are also used as backups, ensuring that a complete set of files is hosted somewhere else in case a server becomes damaged or corrupt in some way.
Classically, mirror sites have been used to fight censorship. A controversial site might be mirrored at a remote location in case the site is shut down, for example, or sites which are banned by censoring software might host mirrors so that people can still access them. A mirror can also serve as a repository for vintage content, a sort of living archive which endures when the original site is taken down or radically redesigned. This can be nice for users who want to see a site in its previous incarnation, or access information which is out dated, but still of interest.