Do you need a use case or a target audience?
A use case means considering how your product will be used by a hypothetical individual who represents a part of your user base - for example, a tech-savvy 35-year-old project manager who is eager to do all her administrative reports in a shared web-based environment instead of working in Microsoft Word and sending the reports as email attachments (note that the use case may be more or less specific). If you're building a product to serve this person, you've got some specifics to work with regarding probable expectations, the amount of guidance and hand-holding needed, and so on. By developing additional use cases describing other probable users (and these are developed by talking with the client - you may even have access to the users themselves) you will have done the groundwork for making a design that suits the users. A target audience is more of a passive recipient, not a true user. "Anyone working in acquisitions" for example. Without a real understanding of the users, the product will be generic and bland. You'll be doing the least you can - putting the product out there in another gray blob among thousands of gray blobs that meet the requirements but are ultimately forgettable. Understanding the difference between a user and an audience is important - are you showing them something or are they doing something?
This distinction is not unique to the web - a hands-on constructivist learning product based on paper or in the classroom can just as easily be dulled into a passive presentation, but web- based products provide constant opportunities to make the choice whether to lecture or to engage. Can the user get in there and really use it -whatever it is - without being told at length what it is, what to do, and how to do it? Less telling the user about the product and more of the user using the product is the goal.