Hi,
All users are mapped with the security role. Security role is contains the access level and privileges for the entities. Please go through the below link it will give you detail view.
https://inventcrm.wordpress.com/2012/09/24/dynamics-crm-2011-business-units-planning-and-implementing-your-organizations-structure/
Thanks & Regards, MS CRM Consultant, V.Surya. My Blog: http://inventcrm.wordpress.com/