Microsoft Knowledge Base Article 891995 contains the following summary and introduction:
SUMMARYThere are two methods that you can use to develop programs that poll the Active Directory Directory service for object attribute changes. This article describes these two methods.
INTRODUCTIONSome database monitoring programs are designed to maintain consistency between object data that is stored in the Active Directory directory service and object data that is stored in another database. The other database can be another Active Directory database, a Microsoft SQL Server database, or some other directory service database. The database monitoring program must track changes to Active Directory object data, such as updates or deletions, and replicate those changes back to the other database.
Database monitoring programs can track changes in Active Directory with either of the following techniques:
|•||Change notification registration|
This article describes the two methods that are available for polling Active Directory. This article does not describe how to use change notification registration for polling Active Directory changes.