The executive producer, the producer, director and the starring actors in a film are what’s called “above the line”. That means that in the budget, they are the people whose fees (which are usually in terms of a percentage) partially or totally come from the gross amount of money the picture makes.
Everyone else is “below the line”. They are hired at a set salary for the duration of their term of employment, and their pay is part of the budgeted costs of the picture.
An executive producer is the person who either brings the project to the studio or is extremely instrumental in getting a picture made, even if they aren’t doing the day-to-day work. e.g., Tyler Perry and Oprah Winfrey were named as executive producers of Precious after the film was made; their names gave the picture some cred in the indie film world and got it shown at Sundance so that a distributor would pick it up.
A producer is the person who does the day-to-day work of overseeing the production of a film from the time the studios give the green light and put up the cash to the picture’s release. They bring the talent and the money together, and they hire all the upper-level employees, like the director, the cinematographer, the set designer, etc. (usually, they like to bring people on they’ve worked with before and have good relationships with, who in turn bring on the slightly lower-level people they like to work with, and so on down to the Production Assistants), approve all the actors cast and approve how the money’s going to be spent. If things go haywire, the studio will step in (or maybe the executive producer), but usually, producers are given enormous leeway in making decisions.
Sometimes the executive producer(s) is also the producer, but usually not. In television, an executive producer is now called the showrunner. Many times, the showrunner created the show, like Joss Whedon (Dollhouse), Stephen Moffit (Doctor Who) or Shonda Rhimes (Grey’s Anatomy), but not always, especially after the creator leaves. Usually, the showrunner is also the head writer.