There is a hidden list in SharePoint which stores the User Details like Name, Email, Login Name etc.
We can access this list using the URL:
Here, when ever a new user is added to the sharepoint site, firstly that user will be added to this list. Hence, we can consider this list as a resource of all users present in the site.
We can acess this list through object model as:
SPList hiddenUserList = SPContext.Current.Web.SiteUserInfoList;
Note that this list contains the users which are deleted from Active Directory too. Hence, whenever we delete a user from SharePoint site, its actually a soft delete and that user details still exist in this hidden list.
SharePoint Rocks !!!