Explore several new features for authentication and authorization in Microsoft Office SharePoint Server (MOSS) 2007 and Windows SharePoint Services 3.0 that make it easier to develop and deploy solutions in Internet-facing and extranet environments.
Have a look at these great articles by Steve Peschka of Microsoft
Forms Authentication in SharePoint Products and Technologies (Part 1): Introduction
Forms Authentication in SharePoint Products and Technologies (Part 2): Membership and Role Provider Samples
Forms Authentication in SharePoint Products and Technologies (Part 3): Forms Authentication vs. Windows Authentication