An Enterprise Service Bus (ESB) is a shared messaging layer that gives you a consistent, scalable and flexible means of coordinating across disparate, loosely-connected services to execute business processes. Over the years, Microsoft has developed several service bus technologies:
BizTalk: A messaging and workflow orchestration platform to build ESB behaviors and capabilities. The BizTalk ESB toolkit provides a set of guidelines, patterns and tools.
Windows Azure Service Bus (ASB): This provides the messaging infrastructure for applications that live in the cloud, in your data center, and crosses all devices and PCs.
Service Bus for Windows Server (SBWS): SBWS is based on ASB and shares many of the same capabilities, such as queue, topic and subscription support. A distinct design goal is to ensure symmetry between SBWS and ASB and allow for a single set of code to be leveraged across both deployment environments.