Information only: this is simply an extension of your business card. It allows you to show off what skills you have and what you have produced to date.
A collaborative or social site: you invite people to join in and contribute, either to showcase their own work or produce a collective work. WritersInTouch.com is a good example of such a site.
A blog: offering news, reviews, and general gossip.
A marketing tool: this sort of site is used to directly market your book. To do so you need to offer more than just information about your book/work and a blog, you need to offer some quality content/articles that people want. This is the sort of site I have opted to develop. I have decided to offer articles on novel writing in Nick Travers Writing Tips, articles on e-promotion for your book in How To Promote My Book On The Internet, and this blog under Nick Travers On Writing, as well as offering articles and background on my book, Helium3. All for offerings are tied together with the NickTravers.com website as a front door. Any ‘sales’ (free download) of the book will be achieved on the back of those offerings. Later, once I have built up an audience, I hope also to offer some sort of collaborative/social element to help coach young writers.
What sort of site you decide to build will depend on how you wish to use the site and the amount of day-by-day effort you intend to devote to running it. This is worth spending some time thinking about as it may well influence where you build your site, who hosts it, and how much you should pay for it.