What is DRBD
DRBD is a block device which is designed to build high availability clusters. This is done by mirroring a whole block device via (a dedicated) network. You could see it as a network raid-1.
DRBD is copyright by Philipp Reisner, Lars Ellenberg and LinBit.
What is the scope of drbd, what else do I need to build a HA cluster?
DRBD takes over the data, writes it to the local disk and sends it to the other host. On the other host, it takes it to the disk there.
The other components needed are a cluster membership service, which is supposed to be heartbeat, and some kind of application that works on top of a block device.
A filesystem & fsck.
A journaling FS.
A database with recovery capabilities.