Forms Based Authentication (FBA) allows a web user to access a SharePoint site and get access to the relevant permissions normally imposed using the groups and permissions within SharePoint. This is basically done by creating a database outlining the necessary roles and permissions that SharePoint can lookup to define what sort of permissions the user has when they logged in externally.
Here are some good links that will help you configure FBA in SharePoint 2010:
Set up FBA with Active Directory as a Membership Provider: http://blogs.msdn.com/b/sridhara/archive/2010/01/07/setting-up-fba-claims-in-sharepoint-2010-with-active-directory-membership-provider.aspx
Set up FBA with AspNetSqlMembershipProvider (SQL Server) as a Membership Provider:
http://blogs.msdn.com/b/sridhara/archive/2010/01/15/setup-claims-using-aspnetsqlmembershipprovider.aspx
Additionally, some tools that could be of help to you:
Directly update the web.config files: http://fbaconfigmanager.codeplex.com
Update the web.config files via a timer job: http://blogs.technet.com/b/speschka/archive/2010/07/28/sharepoint-2010-forms-based-authentication-configuration-manager.aspx
Or if its all too hard and if you have $1300 to spare: http://www.sharepoint-forms-authentication.com