> Legal professionals
Pro bono legal work, which is unpaid, is actually a requirement for practising lawyers in some jurisdictions. New York is one such example.
> Often at least some members of a board-of-directors for a charity will be volunteers
Depending on how you define the terms, all of the directors will be volunteers.
The most common structure for charities is a board of trustees/directors between around 5 and 10 members, who are all unpaid and part-time. The board is led by the Chair(man, woman) of the Board, who is otherwise just an ordinary board member. The meetings are facilitated by the Chief Executive, who is not a board member but a full-time, paid employee of the charity.
Underneath the Chief Executive are other full-time paid staff who are supposed to follow the leadership of the Chief Executive, and who often don't get to attend the board meetings. This group may be any size between zero and thousands of individuals! Confusingly, some organisations call certain people in the middle-management of this team 'directors'.
To conclude:
Board: unpaid - '(Non-executive) Directors', 'Trustees' or 'Councillors'.
Staff: paid, usually full-time - includes 'Executives', '(Executive) Directors' and possibly many other non-management roles.
Some charities' boards get so large they become more like the parliaments of liberal democracies - Speakers, votes, elections.
That's only the most conventional structure - I'd be happy to elaborate on the edge cases too! :)