OK. Let me try...
Advisors: Character with some form of special knowledge, who are willing to divulge their knowledge upon you using the Advisor Action (Free/Court).
All regents get 1 Advisor Action as a Free Action,o
once per turn. Any other Advisor Actions use a court action.
Hirelings: Use Ply-Trade Actions. They typically act as administrators, or some other special skill.
Conceptually, Hirelings are nearly always a part of your domain structure somehow; they have knowledge about your domain and can act in limited ways on your behalf, think if you had a lawyer or a family accountant.
An advisor though, need not be, and can be thought almost like you calling a friend and asking their opinion.