RAC is the principal component for the Oracle Grid Architecture. Oracle’s Grid vision represents a pool of database servers, storage, and networks in an inter-related resource platform. Effective Management of workload within the grid database computing arena is a key feature. RAC Database and clustering technology helps to set up and manage the overall Oracle Grid strategy. Database processing and capability can be easily and dynamically added with the RAC technology.
Keep in mind that Clusters and Grids have different vision and objectives. Clusters have static resources for a specific application. Grids, which can consist of multiple clusters and stand alone servers, are dynamic resource pools and they are shareable among many different applications and users. A grid does not assume that all servers in the grid are running the same set of applications.