What are the Duties of a host?

What are the Duties of a host?
They orchestrate the preparation of everything (if they don't do it themselves, they make sure other people are doing it) such as food, drinks, cleaning, music, table-setting, planning where people will sit or whether they will just mill around all evening. Then once people show up, they greet everyone, take their coats, make them feel welcome, introduce them to others and mention a common interest if they can think of one, steer conversations in good directions if they start going down an unappetising path, watch everyone to see if they're all right or they want something, and in general keep the food and drink flowing. There are whole books on this; search for books on throwing dinner parties or any other parties you're interested. It sounds hard but it just takes a knack for figuring out what people want, which is a really good skill to learn anyway.