A "User" in ServiceBox is any employee, no matter their job ("Role") at the company, who logs in to your ServiceBox site.
A "Role" is defined by the security access that is set up for them, to access only those parts of the site you need them in. The access they are given is so that they are not over-whelmed by all that the ServiceBox site is able to do - given their defined job/"Role"/activity in the company. A "Role" may be as specific so that only one "User" may have that "Role", or be of a generalized nature that many employees may share the same "Role".