LinBit's system for mirroring filesystems across the Network, typically a dedicated LAN, written by Philipp Reisner and LarsEllenberg. This is the general replication technology which is most commonly used by members of by the Linux-HA community. By the way, DRBD is a registered trademark of LinBit.
A background paper by Lars on DRBD 8.0.x is recommended.
For a good overview of DRBD (0.6.x), see Lars's article on DRBD in the November, 2003 issue of the US publication Linux Magazine.
This article has been reviewed for DRBD 0.7: DataRedundancyByDrbd
For further information, refer to the drbd.org site, and the DRBD pages in this wiki:
If you want to be on the safe side, or just want to support the authors, commercial DRBD support is available.