A cluster is a collection of servers (called nodes) operating as one unit. A cluster is useful for high performance and high availability. If your nodes cannot handle the tasks, add more nodes, which adds more resources to get the job done. If a node fails, the work can still continue on the other nodes.
Real Application Clusters (RAC) is Oracle's clustering software for its database. With RAC, you can have multiple nodes in the cluster.
This was first published in April 2006