Producer
The chief of staff of a movie production in all matters save the creative efforts of the director, who is head of the line. A producer is responsible for raising funding, hiring key personnel, and arranging for distributors.
Associate Producer
An individual who performs a limited number of producing functions delegated to her/him by a producer, under the direct supervision and control of that producer. The term may also refer to a person who would qualify as an executive producer of a project, but for the fact that (s)he acts on behalf of a production company which is subordinate to another one on that project.
Executive Producer
A producer who is not involved in any technical aspects of the filmmaking process, but who is still responsible for the overall production. Typically an executive producer handles business and legal issues.
Line Producer
A producer who is responsible for managing every person and issue during the making of a film. Line producers only work on one film at a time.
All the above is a simple cut-and-paste from the IMDb Movie Terminology Glossary