Heartbeat is a daemon that provides cluster infrastructure (communication and membership) services to its clients. This allows clients to know about the presence (or disappearance!) of peer processes on other machines and to easily exchange messages with them.
In order to be useful to users, the Heartbeat daemon needs to be combined with a cluster resource manager (CRM) which has the task of starting and stopping the services (IP addresses, web servers, etc.) that cluster will make highly available. Pacemaker is the preferred cluster resource manager for clusters based on Heartbeat.
Since up to release 2.1.4 the messaging layer (Heartbeat proper), the Local Resource Manager, "plumbing" infrastructure and STONITH (now known as Cluster Glue), the Resource Agents, and the Cluster Resource Manager (now Pacemaker) were all part of a single package named
heartbeat, the name was often applied to the Linux-HA project as a whole.
This generalization is no longer accurate, the name heartbeat should thus be used for the messaging layer exclusively.