Oracle Clusterware is the software which enables the nodes to communicate with each other, allowing them to form the cluster of nodes which behaves as a single logical server.
The CRS service has four components, each handling a variety of functions: Cluster Ready Services daemon (CRSd), Oracle Cluster Synchronization Service Daemon (OCSSd), Event Volume Manager Daemon (EVMd), and Oracle Process Clusterware Daemon (OPROCd).