In television, the executive producer is the person who came up with the project/put his or her name or money into the project/brought the idea for the project to the network/studio. In film, it’s a very similar concept.
For example, Matthew Weiner came up with the idea for Mad Men. When AMC decided to go with it, he became the executive producer, as well as head writer.
The producer is the person who actually does the work of getting the idea together with the right talent, staff and crew and oversees everything.
People who are the executive producers, head writers and overseers of the production all at the same time are called showrunners. Matthew Weiner is a showrunner. Stephen Moffat (who is going to be the new head writer and executive producer of the upcoming series of Doctor Who in 2010) is a showrunner. This title is used for television only.
The producer has line producers (this is how far I’ve gotten at present and could use a job!) and production managers under them to make sure that the day-to-day tasks are carried out (line, assistant and associate producers) and that nothing goes over budget (production managers).
In radio, the executive producer is usually someone high up at the radio station that is producing the show, and is very often the host of the show. The producer has the same function as in TV and handles the day to day tasks, which in radio is much more of a hands-on job. There are no showrunners in radio.