(1) in Canada and the USA, a producer is the person responsible for the financial and administrative production; the person who exercises general supervision of a production and is responsible chiefly for raising money, hiring technicians and artists, etc. required to make a film or stage a play; (2) in British (& Canadian) Television, film and Theatre, the producer was the director of the theatrical production (i.e., stage director)...today, the Canadian and British film industry of adopted the American usage.
The person who (in theory) brings together all the elements to make a film. A producer shepherds an idea from scripting through the theatrical release, hiring and firing personnel as needed. The "Best Picture" Oscar goes to them. For information on the different types of producers, see the Producer page.