The Back of the Napkin: Solving Problems and Selling Ideas with Pictures, by Dan Roam.

"You see, Freddie," I said, "Lots of people these days are very worried and confused about how to create a useful website. But the way I think about it, there are really only three things that we need to define. The first is the brand itself. The other two are the content and the function." Then I drew in two more circles and labeled them appropriately, then continued, "If we can determine what to put in these three circles, then we can build any site to serve any audience."
"The question is: how do we know what these three should contain. The answer is this." I drew a little smiley face next to each circle and wrote a caption for each. "What people want to DO (or what we want them to do) determines function; what people want to KNOW (or what we want them to know) determines content; and what we want them to REMEMBER determines the brand."

Quoted from : 800CEOread blog