Where does SPUser.ID comes from

Dear All

I have a user set up in active directory. This user is rwolf (CN=rwolf,OU=acme,DC=bfs-cust-tst,DC=local). Assume I run the code below in  'protected void Page_Load(object sender, EventArgs e)'.

SPSite oSite = SPContext.Current.Site;
SPWeb oWebsite = oSite.OpenWeb();
SPUser user = oWebsite.CurrentUser;
myLabel.Text = user.ID.ToString();

When rwolf logs into Sharepoint and navigates to the page where above code behind runs he will see as value in 'myLabel' '18'. So, ID of rwolf is 18. Now, I could not figure out where this ID comes from. I looked deeply into Active Directory but could not find anything. Any hints?