When you're getting started, it's important to understand all you can about the situation for which you're creating or overhauling the website. You're unlikely to get the full picture though, however hard you try.
"In lieu of perfection, learn whatever you can about the objectives, requirements and unrequirements of your user and business, and use this intelligently in your design."Definitions paraphrased from the book:
Objectives - answering the question, "What do we want to achieve?" Good objectives statements begin with words like reduce and increase, indicating where you want to be relative to where you are now.
Requirements - answering the question, "How should we do it?" and represent a balancing act between what the user wants to do and what the business needs to achieve.
Unrequirements - the "how not to"which goes hand-in-hand with requirements' "how to". So we're talking about constraints (the usually immovable things in all projects that come in the form of technology, regulations, deadlines and the like) and exclusions (the approaches that the business deliberately wants to avoid for whatever reason).
Undercover User Experience Design - a recommended addition to your bookshelf.