All the directors together are the board. They meet in the boardroom.
Non-executive directors are not managers of the company; they are outsiders, often directors of other companies who have particular knowledge of the industry or of particular areas.
The marketing director is the head of marketing; the IT director is the head of IT, etc. These people head or head up their departments. Informally, the head of an activity, a department or an organization is its boss.
An executive or, informally, an exec, is usually a manager at quite a high level (for example, a senior executive). But 'executive' can be used in other contexts to suggest luxury, as in 'executive coach' and 'executive home', even for things that are not actually used by executives.