The Real Application Cluster (RAC) component allows for the deployment of a single database across a cluster of servers. This configuration provides a high degree of fault tolerance (if one server fails, another can assume its workload) and scalability, as servers can be easily added or subtracted from the cluster to meet current needs. Additionally, RAC can lower costs because companies are not required to buy high-end servers.
For more information on RAC, see the Oracle Real Application Clusters document at http://www.oracle.com/us/products/database/options/real-application-clusters/overview/index.html.