High availability is the implementation of technology so that if a component fails, another can take over for it. By using highly available platforms, the downtime for a system can be reduced, and, in many cases, it can be reduced to a short enough time that the users of the system do not see the failure.
HA clusters are often used for critical databases, file sharing on a network, business applications, and customer services such as electronic commerce websites.
HA cluster implementations attempt to build redundancy into a cluster to eliminate single points of failure, including multiple network connections and data storage which is multiply connected via Storage area networks.
If you want to know more how to set up and configure iStorage Server for High-Availability SAN, please refer to those White Papers:
Windows Server 2003
Windows Server 2008