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 such a site, ranging from a desire to ensure that people have access to information to an attempt to plagiarize. 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 website.
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.
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 website, 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.
Software downloads are often hosted on mirror sites to avoid overwhelming the server, and for the convenience of users. For example, a download site based in Germany might offer a mirror site based in Japan for Japanese users, making it faster for them to download files. The distribution of the software across a number of servers also ensures that it will always be accessible to users, even if one or more sites go down.
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 dated, but still of interest.