As I understand it, an Executive has voting rights and is involved in making decisions that affect the company, a Non-Executive has no such rights, but sits on the board in an advisory capacity.
His role is fairly restrictive and probably part time. He will attend board meetings and may sit on some specialist committees (often renumeration committee which decides directors' salaries). He will have no day to day responsibilites to do with the running of the business. I believe it is a legal requirement that PLCs have non exec directors.