WinDRBD is a Windows kernel driver that provides (almost) the same functionality as DRBD under Linux. It is based on DRBD 9 and uses the original DRBD source code wrapped in a compatility layer that makes it possible to run DRBD within Windows NT kernels.
By using WinDRBD one can implement high availablility (HA) on Windows servers or even mix Linux and Windows servers. In such a scenario, if a node fails another node can take over the services (like databases, web servers and so on). This makes it possible to achieve almost 100% availability for internet servers.
As the image outlines, WinDRBD translates DRBD API calls (which are Linux kernel specific) to Windows NT kernel API calls. This is done for storage, networking and user node level interfaces (resource configuration via netlink and providing storage interface for the WinDRBD resources).
Also shown is that WinDRBD can talk seamlessly to Linux DRBD which makes mixed Linux/Windows setups with up to 32 nodes possible.