This represents the collection of users or user objects who have been explicitly assigned permissions in the Web site . This does not return users who have access through a group.
SPUserCollection users = web.Users;
This gives us the collection of user objects who are either members of the site collection or who have atleast navigated to the site as authenticated members of a domain group in the site.
SPUserCollection users = web.AllUsers;
This is the collection of all users that belong to the site collection.
SPUserCollection users = web.SiteUsers;
So I thought it would be nice to write an article stating which is what. Hope this information helps you SharePoint 2007 developers.