I'm confused little bit about Facebook Connect.

Looking this from technical perspective, when users are created usually a user ID is saved in a database and used later on the website to identify users, but using Facebook Connect how do I identify users.

For ex. A user logs in on my website using Facebook Connect, and he wants to upload a file on my website.

How my application knows this user?
Do I have to import Facebook Connect user data and create a new record in my database?