What is the diffrence between AllUsers, SiteUsers, Users ?

In the SPWeb object you have those three properties that sounds a like, but there is a slight diffrence between them -

AllUsers - Returns "almost" all users in the site collection ( including "guests" ).

SiteUsers - Returns all users that are really exist in the site collection, including groups such as "authenticated users" and system accounts and etc.

Users - Returns only the users that appear in that specific web site.

The explanation in the SDK wasn't clear enough ( as for now , before the service packs ).

If you want to check it yourself just run this short code and see the difference.

 

//Open a SPWeb object ( a site collection )

using (SPWeb web = colSites[i].OpenWeb())
                    {
                       SPUserCollection colUsers =  web.Users;
                       foreach (SPUser user in colUsers)
                       {
                           Console.WriteLine(user.Name);
                       }
                       Console.WriteLine("=======================");
                       SPUserCollection colUsers1 = web.SiteUsers;
                       
                       foreach (SPUser user in colUsers1)
                       {
                          Console.WriteLine(user.Name); 
                       }
                       Console.WriteLine("=======================");
                       SPUserCollection colUsers2 = web.AllUsers;
                       
                       foreach (SPUser user in colUsers2)
                       {
                           Console.WriteLine(user.Name);   
                       }
                        
                        
                    }