With the advent of Web services and SOA, we've been seeking to create architectures and systems that are more loosely coupled. Loosely coupled systems provide many advantages including support for late or dynamically binding to other components while running, and can mediate the difference in the component's structure, security model, protocols, and semantics, thus abstracting volatility.
This is in contrast to compile-time or runtime binding, which requires that you bind the components at compile time or runtime (synchronous calls), respectively, and also requires that changes be designed into all components at the same time due to the dependencies. As you can imagine, this type of coupling makes testing and component changes much more difficult.
