Since the beginning of computing we've been dealing with the notion of coupling, or the degree to which one component is dependent on another component in both the domain of an application or an architecture. Lately, the movement has been towards loose coupling for some very good reasons, but I'm not sure that many architects out there building SOAs understand the motives behind this.

Breaking this concept down to its essence, we can state that tightly coupled systems/architectures are dependent on each other, thus changes to any component may prompt changes to many other components. Loosely coupled systems/architectures, in contrast, leverage independent components, and so can operate independently. However, it's not as simple as all that.

Keep in mind that how loosely or tightly coupled your architecture is is a matter of requirements, and not as much about what's popular. Indeed, architects need to understand the value of SOAs and loose coupling, and make the right calls to insure that the architecture matches the business objectives. So it's helpful to walk through this notion of coupling as you approach your SOA.