There are the “film production” rights and there are “film distribution” rights.
Film production rights are the most important ones, as they’re the ones that dictate whether you can actually, you know, produce a film with an intellectual property. Marvel Studios currently has the film production rights to Captain America, Thor, Iron Man and the Guardians of the Galaxy, while Sony owns the film production rights to Spider-Man and Fox owns the film production rights to the X-Men and the Fantastic Four. Marvel owns the film production rights to the Hulk, which is why they’re able to use him in the Avengers.
Film distribution rights, however, are a whole other thing that is still quite important. Distribution rights are the right to actually put the film into theaters. They make the film prints of the film, they make the deals with the theaters and they make the final call on the release date of the film and how the film is relased (video on demand, etc.). Not a lot of people out there can actually distribute a film. It is a very expensive undertaking, so pretty much only the major film studios can afford to do it. Marvel Studios, when they first started in the business of producing their own films, could not afford to do it. Therefore, they worked out a deal with Paramount Studios to distribute their Marvel Studios films such as Iron Man, Iron Man 2 and Captain America: The First Avenger. The distributor gets a percentage of the film’s box office (plus expenses typically) in exchange for distributing the movie. Universal Pictures, however, has a different deal. They initially had the film production rights on the Hulk, as they produced the 2003 Hulk film. The film production rights eventually reverted to Marvel, who produced 2008’s Incredible Hulk. Universal, though, continued to distribute the film under a deal where Universal would get the first chance to distribute any future Hulk movies.
