be aware the link that jackpal gave you is for an older version of samba, and will be of limited use. a better link using samba 3 is:
to answer your questions:
1. if you did not have winbind, then in order for each domain user to access you linux share, you would have to add that domain user as a local user to your linux server. obviously, a lot of work in a large organization. so winbind adds all of the domain users to the user list of the linux server.
2. the link above will show you how to join the domain, and set up a share on your linux server that all domain user can access.
3. if you want a domain user to be able to physically login at the linux server using their domain name and password, i have not figured that one out yet. It has to do with authentication, using either PAM or Kerberos.
please, out of courtesy, write back to close this thread by telling us that the solution worked, did not work, or you solved it by...
people that leave threads open do not get answered in future threads.