Do they get paid by shares or salary? I mean is it a viable alternative to having a "job"? I see many people around sitting on multiple boards of companies.
If you are a investor in the company you usally don't get paid, just your trip expenses.
If you are an external member, somebody strategic to the company, you get paid either by meeting or some shares. Usually all board members in public companies are paid with cash.
Absent an alternative structure, default corporate laws provide for the directors of a company with only one class of common stock to be elected by majority vote of the stockholders.
However, there are all sorts of alternative structures that can alter this default. For example, it is common in a VC-backed company that preferred stockholders are given the right to designate a specified number of board members. The right is usually laid out in the company's certificate of incorporation or a voting agreement among the stockholders.
Source: I am a startup attorney.