Todd Dickerson explains why websites fail at sales: they give visitors too many choices, while funnels guide them step-by-step toward a purchase. A funnel controls the path. A website just opens doors.
Chapters
I would say the difference between a funnel and a website is that you are directly leading your customers or your prospective customers on a journey to exactly what you want them to see. So instead of just showing them this page with a bunch of options, you're saying, I want you to see this specific thing, and then I want you to click on this, and then I want you to see this next specific thing, and then I want you to put in your credit card information, and then I want you to see this next thing, and you're really able to control that behavior and make sure that they are seeing what you want them to see in the sequence that you want them to see it in. That's how I would describe it. But you should probably read Russell's books, because he describes it like the best.