There is no difference between backend users and other users saved with the user. This difference is done with user groups.
Also the private information to each users are not saved with the user. They are hold in the user's profile.
The following information are saved for users:
the username (Login)
the password (is not shown in modify mode); if it shouldn't be changed leave it empty
groups, the user is can be a member of
block a user
mark user as deleted
status if account is activated
If field status is inactive or field blocked is yes or field deleted is yes the user can not log in.