Such sites have different URLs than the original site but host identical or near-identical content. Mirror sites are often located in a different geographic region than the original, or upstream site. The purpose of mirrors is to reduce network traffic, improve access speed, and ensure the availability of the original site for technical or political reasons, or to provide a real-time backup of the original site. Mirror sites are particularly important in developing countries, where internet access may be slower or less reliable.

Mirror sites were heavily used on the early internet, when most users accessed through dialup and the Internet backbone had much lower bandwidth than today, making a geographically localized mirror network a worthwhile benefit. Download archives such as Info-Mac, Tucows, and CPAN maintained worldwide networks mirroring their content accessible over HTTP or anonymous FTP.

Some of these networks, such as Info-Mac or Tucows are no longer active or have removed their mirrored download sections, but some like CPAN or the Debian package mirrors are still active in 2023. Debian removed FTP access to its mirrors in 2017 because of declining use and the relative stagnation of the FTP protocol, mentioning FTP servers' lack of support for techniques such as caching and load balancing that are available to HTTP. Modern mirrors support HTTPS and IPv6 along with IPv4.

On occasion, some mirrors may choose not to replicate the entire contents of the upstream server because of technical constraints, or selecting only a subset relevant to their purpose, such as software written in a particular programming language, runnable on a single computer platform, or written by one author. These sites are called partial mirrors or secondary mirrors.[