A backend server suite from Microsoft. From the horse's mouth:

BizTalk is an industry initiative started by Microsoft and supported by a wide range of organizations, from technology vendors like SAP and CommerceOne to technology users like Boeing and BP/Amoco. The full BizTalk initiative comprises two separate areas:

The first is the open, platform and protocol independent BizTalk Framework. This framework is basically a set of recommendations on how to use XML in an open way, which makes it easier to exchange business documents. The BizTalk Framework is supported by the web site http://www.biztalk.org/, where companies can find or submit XML schemas.

The second part of the BizTalk initiative is the development of tools and servers to support the exchange of XML messages in a loosely coupled way. For instance, Microsoft is developing tools for building XML applications, including the Microsoft BizTalk Server that provides reliable message transfer.