SharePoint technologies can use multiple authentication providers and if your brave enough you can write your own (If you have modified your LDAP schema or have a non-supported environment). If you are configuring SharePoint for internal or external use, there are many whitepapers that cover how to chose a provider that suits your situation and how to configure it.
Before you start, make sure you understand your companies data and security policy because in some cases internal and external users and data cannot exist on the same systems. So you might require separate farms (SharePoint and SQL) or at a minimum SSPs.
Here are some links to help you get started:
- Active Directory Application Mode (ADAM)
- SharePoint ADAM Role Provider
- Step-by-Step Guide to Deploying ADAM
- How to use WSS v3 with ADAM for user authentication
- SharePoint People Picker Behavior with ADAM Users
- Create your LDAP data store with the Active Directory Application Mode (ADAM) directory service