An "executive producer" is someone who is either financing a film, or is representing a studio or party that is financing a film. Films can have multiple backers, and therefore more than oneexecutive producer. Executive producers -- often referred to as "the suits" because of their formal attire -- may not have any movie-making experience at all.
When a studio invests in a motion picture and it assigns an executive to oversee the making of the film, this executive is given the title executive producer. But he or she doesn't have a specific job on the set. Instead her responsibility is to make sure that everyone else is doing their job -- that the project is on schedule and is not over budget. The executive producer protects the studio's investment by overseeing the project.
The executive producer will work closely with the director if any concerns arise. For example, if bad weather holds up filming, or an actor is injured -- if anything at all goes wrong that threatens the picture staying on budget or on schedule -- the executive producer will press for solutions.
Another role of executive producer is to make sure that as the film is being made as planned; they ensure that ad hoc changes do not inherently alter the original project the studio approved.