The USER is the basic user of the HD module.
He is the one assigned tickets for the issues for which he is requesting help from the helpdesk, and can issue HD Ticket Followupss to them.
He is is bound to a Customers, who is the one actually managing the support contracts. Of course, any many cases, a user will also be a customer, so the roles and permissions will be combined.
Depending on setup, users may or may not be allowed to create their own tickets. For corporate-type support, tickets will typically be created by the person registered as customer, who will create the tickets on their behalf.