Yes, SharePoint supports the .NET 2.0 Membership Provider Model. You can store the user accounts in SQL Server, Active Directory, ADAM or any other LDAP store by configuring one of the providers supplied with SharePoint (It uses Active Directory by Default but others can be configured)
It is extensible as well. If one of the MS supplied providers doesn' tmeet your needs, you can extend or replace it by writing your own provider.
Hope that helps