Producers come in a variety of types and have different roles in production. Damien Parer sums up the different types of producer quite well in his “Glossary of FilmSpeak” in Film Business: A Handbook for Producers.
Producer
The person responsible for the film from concept to maximisation of revenue. The person in authority to make artistic and financial decisions. The entrepreneur who causes the film to made and released.
Executive Producer
Usually the person who has arranged finance for the film. The title is also sometimes used for star’s agents, people who finalise presales, rather important contributors to the production – anyone really. A good example of the later is George Lucas” somewhat unique position on The Empire Strikes Back and Return of the Jedi. Obviously Star Wars was Lucas” baby, so he had important input into the editing and direction of the film, even though these roles were filled by others.
Line Producer
The person who takes responsibility for the production of the film. Line producers are generally employed just before pre-production and complete their work at the answer print stage.
Co-producer
It could mean what it implies or it could mean anything. It seems to be between the credit of Line Producer and Producer. It also may be shared producer responsibility.
Associate Producer
On larger productions, it is sometime necessary for the duties of the producer to be divided up amongst several people. Where someone more junior is taking on producer duties, they are often credited as an associate producer. This title is also sometimes give to a person who has made a major contribution to the production. It could be a financier, production manager, writer, post-production supervisor, actor, etc.
The Producers’ Guild of America, and similar organisations in other countries (e.g. PACT in the UK or SPAA in Australia) are actively working to define more clearly the type of roles on a film project which are worthy of a producer credit.