Linbit's system for mirroring blockdevices (filesystems, VM images, whatever) across the Network, typically a dedicated LAN, but also cabable of WAN replication, written by Philipp Reisner and Lars Ellenberg
This is the general replication technology which is most commonly used by members of the Linux-HA community. By the way, DRBD is a registered trademark of Linbit.
The DRBD User's Guide is excellent documentation and reference, you are strongly encouraged to thoroughly read it. Learn how to set it up manually and how things work. Use DRBD manually to get a feeling for what it does. Then you may start thinking about using it as a cluster resource.
For further information, refer to the drbd.org site.
If you want to be on the safe side, or just want to support the authors, commercial DRBD support is available.