A mirror is a website that literally reflects the original website. Many websites need mirrors, due to the realities of the internet and its uptime and latency follies. A mirror must be updated by access to the original. It can be done through the HTTP, FTP or SSH protocol, depending on the urgency of the situation.
The mirror can be configured to come on automatically via a DNS entry and a failover entry in the DNS record. This is probably the best practice for very critical websites, like PayPal, Google, or Microsoft. The main advantage is latency. So, for example, if someone from California requests a page at Microsoft, it’s best that the person be directed to the closest California server; in this way, the user would have the fastest response time.
Not only is it important for the user to get the fastest response time, it is also important for the user to get the most up-to-date and appropriate information. Therefore, having a coordinated mirror network is most useful. That way, each mirror server would be synchronized from the master server at very short time intervals, or based on new information as it happens. That way, the mirror server would get the latest information and provide the fastest response time to each user.
Mirrors are mainly used to provide download locations for operating systems like Linux and its different distributions. Operating systems are not updated frequently, that way one can support the operating system for a very low cost, all over the world and lend a helping hand to the progress of the cause supported by the operating system. For example, parsix is an operating system that is made especially for people who speak Farsi. That may not be the most profitable proposal per se, but it certainly fulfills a social objective and, if someone has excess resources, the most appropriate thing would be to support that cause. By providing them with bandwidth and disk space, anyone with an Internet connection can provide them with a mirror site.
Mirrors should not only be of an operating system, but can also be of other information, such as large files that users need to download, images that are very heavy in resolution, etc. Providing a mirror always helps the mirroring site improve its user base, latency, and overall uptime.