High Scalability Building bigger, faster, more reliable websites.

From their website:
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 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.