There are many jobs and roles played by many different people when creating a film. There is so much going on behind the screen aside from what you actually see. Whether these are very major, or very minor roles, each has its own significance in the film making process.
Almost certainly the most essential person to the film would have to be the producer. This is the person who is in charge of basically all artistic and financial decisions. They are more or less the "entrepreneur" who causes the film to be released. This person will have all the last says regarding the production of the film. When all's said and done there would really be no film without this person in the beginning of it all.
The director is another very major part in the making of a film. The principal role of the director is to take the film from the script and put it onto the screen.
The director picks the actors and actresses that will be in the film. This person is essential in making the film what it is going to be. Without a good director, you will have a bad movie guaranteed. But not every director can create any type of movie, no matter how famous or acclaimed they are.
Director of photography, this person is responsible for all the lighting and types of lenses and whatnot that the cameras use. This individual works closely with the director and has another very major role in the film.
Executive, co, line, and associate producers all have their own specific jobs but basically all just work under the producer and have more specific duties of his.
The person who's responsibility it is to keep track of everything such as number of scenes covered, scenes taken, lunch...