The Linux-HA project maintains a set of building blocks for high availability cluster systems, including a cluster messaging layer, a huge number of resource agents for a variety of applications, and a plumbing library and error reporting toolkit.
You usually want to deploy the Pacemaker Cluster Resource Manager on top of the Heartbeat infrastructure layer. If you happen to be still under the impression that this CRM would be part of Heartbeat, please see Why was the Pacemaker Project Created?
DRBD® refers to block devices designed as a building block to form high availability (HA) clusters. This is done by mirroring a whole block device via an assigned network. DRBD can be understood as network based raid-1.
In the illustration above, the two orange boxes represent two servers that form an HA cluster. The boxes contain the usual components of a Linux™ kernel: file system, buffer cache, disk scheduler, disk drivers, TCP/IP stack and network interface card (NIC) driver. The black arrows illustrate the flow of data between these components.
The orange arrows show the flow of data, as DRBD mirrors the data of a high availably service from the active node of the HA cluster to the standby node of the HA cluster.