The directors act as a board but the board may (if the articles permit, as they generally will) delegate powers to a committee of board members or to an individual director.
Non-executive directors are, as their name implies, directors to whom no executive powers have been granted by the board. Although they have no executive powers, they can vote at board meetings and have the same duties as executive directors.
Executive directors are generally employees with specific powers delegated to them either by a resolution of the board or under their service contracts.
Most companies have a managing director (sometimes called a chief executive). He is granted more extensive executive powers by the company's articles or by board resolution.