A mirror is a website that literally mirrors the original website. Many websites need mirrors, because of the realities of the internet, and its follies of uptime and latency.
A mirror must be updated by access to the original. It could be done through HTTP protocol, or FTP or SSH, depending on the urgency of the situation.
The mirror can be set up to kick in automatically by a DNS entry, and a failover entry in the DNS record. This is probably the best practice for highly critical websites, such as PayPal, Google or Microsoft. The main advantage is the latency. So, for example if someone from California requests a page on Microsoft its better that the person gets directed to the closest California server – this way, the user would have the fastest response time.
Not only is it important for the user to get the fastest response time, its also important for the user to get the freshest and most appropriate information. So, having a coordinated mirror network is the most helpful. That way, each mirror server would get synced from the master server at very short time intervals, or on the basis of new information as it happens. That way, the mirror server would get the newest information, and provide the fastest response time to each user.
Mirrors are most used to provide download locations for operating systems such as linux, and its various different distribution. The operating systems are not often updated, that way one can support the operating system for a very low cost – across the world and lend a hand of support to the progress of the cause supported by the Operating System. For example parsix is an operating system that is made specially for people who speak farsi. That may not be the most profitable proposition per se, but it certainly meets a social objective and, if anyone has excess resources – supporting such a cause would be most apt. By, providing them with bandwidth and disk space – anyone who has an internet connection can provide them with a mirror site.
Mirrors need not only be of an operating system, they could be of other information such as heavy files that users need to download, images that are very heavy in resolution etc. Providing a mirror always helps the site being mirrored to enhance its user base, its latency and its uptime in general.