When a firm begins the process of an Initial Public Offering, they hire an underwriter (bookrunner) to drum up support for the issue.

When a firm begins the process of an Initial Public Offering, they hire an underwriter (bookrunner) to drum up support for the issue.  This begins with some due diligence, legal research, filing with the SEC, and a road show.  When all is said and done, the underwriter either makes a firm commitment or a best efforts offer to the company that is trying to go public.

What is the difference between the two and why does it matter?