I need some advice about Active Directory

Hello I'm working as a system admin. I have the following two questions about Active Directory:
- When a user logs on to the system using the active directory, they log on to the AD(NTDS.DIT) database. Can you tell me where the AD(NTDS.DIT) database is located ? Is this separate from SQL server database? How do you access the AD(NTDS.DIT) database?
- How do you connect the AD(NTDS.DIT) database to SQL server 2008, if their databases are located at different locations?
Can some one explain the above two questions.
Thank you.
