Steve Blank has an excellent post about it.
A startup is a search for a repeatable and scalable business model. As a founder you are testing a series of hypotheses about all the pieces of the business model: Who are the customers/users? What’s the distribution channel? How do we price and position the product? How do we create end user demand? Who are our partners? Where/how do we build the product? How do we finance the company, etc.
Also check out this excellent talk about Why Accountants Don't Run Startups he gave at Startup Lessons Learned Conference in San Francisco on April 23, 2010 on Customer Development 2.0.