What is Oracle Clusterware?

The Oracle Clusterware was introduced in 10.1 (initially called CRS ) and is the product that lies under RAC. Oracle Clusterware provides different services to RAC including:

Group Services
Node Monitor
Locking services
HA Resource management
Event framework, etc

Components.Several components are needed to have CRS running on Unix machine. Here is a brief description of each:

Daemons and init.* scripts
Oracle Clusterware is formed of several daemons, each one of which have a special function inside the stack. The daemons are located inside the directory $CRS_HOME/bin. Here is a list of the daemons, for 10.2.0.3 and later, note that depending on the platform and whether or not there is a 3rd-party vendor clusterware installed, some of the following processes may not be present:

ocssd.bin
crsd.bin
evmd.bin
ocslsvmon.bin
oclsomon.bin
oprocd

Oracle Cluster Registry and Voting Disk
The OCR contains the configuration information for the clusterware, like the network endpoints where the daemons (ocssd.bin, crsd.bin,etc) will be listening, cluster interconnect information for RAC, location for Voting disk, etc.

The Voting disk is a communication mechanism where every node reads and writes its heartbeat information. The Voting disk is also used to kill the node(s) when the network communication is lost between one or several nodes in the cluster to prevent a split-brain and protect the database information.