The term high availability is the buzz word nowadays. Nobody tolerates downtime, whether it is the owner of a web site, owner of a server or owner of a data center. The challenge is how you can offer the least down time and the term "high availbility", has it all.
DRBD refers to "Distributed Replicated Block Device" and is used for building high availability (HA) clusters. This can be attained by mirroring or replicating a block device via network. DRBD can be considered as equivalent to a RAID 1 setup.