The next version of Samba, version 4, is fundamentally different from the previous version. The configuration options as well as the storage media for various things (password database, account policies, printer data, file attributes, etc) have changed radically.
Samba3 stores pretty much all data in TDBs. The only exception to this rule are the password backends (smbpasswd and LDAP).
Samba4 stores its data either in LDAP-databases, in extended attributes or in custom key-value databases (TDB). Fallback is provided where needed: an embedded LDAP-like database(LDB) is used if there is no LDAP server available, and a custom database is used if the file system does not support extended attributes. A lot of the configuration options available in Samba3 have been obsoleted or moved to databases in Samba4.
As part of the Google Summer of Code program, I have worked on implementing a one-way conversion utility for upgrading from Samba 3 to Samba 4 and Samba4 support for using LDAP servers with Samba 3 schema's.
Thanks to Google for sponsoring!