A mirror site is a website or set of files on a computer server that has been copied to another computer server so that the site or files are available from more than one place. A mirror site has its own URL, but is otherwise identical to the main site. Load balancing devices allow large volume sites to scale easily, dividing work between multiple mirror sites.
A mirror site is often updated frequently to ensure that it reflects the content of the original site. In some cases, the original site can be arranged for a mirror site in a larger location with a higher speed connection and, perhaps, a closer proximity to a large audience.
If the original site generates too much traffic, a mirrored site can ensure better availability of the website or files. For websites that offer copies or updates of widely used software, a mirror site allows the site to handle larger demands and allows downloaded files to arrive faster. Microsoft, Sun Microsystems and other companies have mirror sites from which to download their navigation software.
Mirror sites are used to make access to the site faster when the original site may be geographically distant from those accessing it. A mirrored web server is often located on a different continent from the main site, allowing users near the mirror site to obtain faster and more reliable access.
You can also make a mirror of a website to ensure that information can be made available in places where access may be unreliable or censored. In 2013, when Chinese authorities blocked access to foreign media such as The Wall Street Journal and Reuters, the site's reflection was used to restore access and bypass government censorship.