If your website is suffering from the sluggish performance that comes with high traffic, a dedicated web cluster is the perfect choice. Web clusters come in many varieties, but the purpose of using a cluster is always the same: spread the burden of serving your web content across multiple computers. In addition to faster load times, clustering provides protection against downtime.
With only a single web server a crash means your critical content is offline. In a cluster, another server simply takes over and your customers don't notice a thing.
High availability is an approach used to design systems and their associated service so that a given level of uptime will be met over the desired systems.
While all users and administrators seek maximum uptime, or availability, from the systems they use and administer, certain systems are critical enough that additional effort and money must be spent to minimize downtime. Hospitals, communications, and infrastructure control systems are the most common examples, but high availability systems design is used in many different industries.