The term “high availability” describes a servers system that are optimized for uninterrupted operation with a minimum of unplanned and planned downtime. Usually, this is achieved by avoiding single points of failure and adding redundancy to the entire system. Common solutions are hot-standby systems that take over the role of a crashed server in milliseconds. More sophisticated solutions offer both scalability and high availability by using clusters of servers that run their applications in a distributed manner, this is entirely invisible to their clients.
High-availability clusters
Are computer clusters that are implemented primarily for the purpose of providing high availability of services which the cluster provides. They operate by having redundant computers or nodes which are then used to provide service when system components fail.High-performance clusters
Are computer clusters that are build primarily for the purpose of providing high performance in delivering applications and content. They are not as redundant than the High-availability clusters but can handle more traffic and visitors. Downtime will usually be more important in case of failure.