ASP.net Windows authentication compared to Forms authentication using ActiveDirectoryMembershipProvider

Question

I was wondering if someone could explain the difference between forms authentication using the ActiveDirectoryMembershipProvider, and Windows authentication. When would one be used over the other and what would the benefit be?

I am working on an application which initially will be using the SqlMembershipProvider and ASP.net membership, roles, and profiles. But there is a possibility down the road where authentication will need to be done from Active Directory, and I would like to plan ahead so that the change will result in minimal refactoring. Has anyone had similar experience and could provide insight to approach this?