David Travis of usability company Userfocus does a great job of explaining what it's all about.
Many design teams launch into development without a shared vision of the user experience. Without this shared vision, the team lacks direction, challenge and focus.Does this sound familiar to your situation? However you develop and share your vision, it's critical to a successfully managed website.
Why you need a user experience vision (and how to create and publicise it) - article by David Travis for userfocus.co.uk