An experience theme helps illustrate for your team the kind of experience which separate elements of a website should work to create; the kind of elements you will use to tell your website’s story. Effectively executed, experience themes objectify design and developments decisions which would otherwise be subjective. That alone is worth the upfront cost of introducing experience themes to the design process.
via uxbooth.com
Thinking about how best to use sketchboarding and experience themes. Nice links to many of the relevant sources in this article.