One of the rights you get in a copyright, at least in the U.S., is the right to prepare "derivative works." What is a derivative work? Section 101 of the Copyright Act says:
A "derivative work" is a work based upon one or more preexisting works, such as a translation, musical arrangement, dramatization, fictionalization, condensation, or any other form in which a work may be recast, transformed, or adapted. A work consisting of editorial revisions, annotations, elaborations, or other modifications which, as a whole, represent an original work of authorship, is a "derivative work."
Of course, you can authorize someone else to make derivative works based on your work: this is how Michael Crichton and Tom Clancy bring in the big bucks from movie deals. When someone else prepares an authorized derivative work, they get their own copyright in whatever contribution they make to the work.
Take Jurassic Park, for instance. Crichton wrote the novel, and therefore has the copyright in the book. By extension, he has copyright in the characters, the plot, and every other creative feature that appears in the book. After he authorized Steven Spielberg to make a movie out of the book, Crichton continued to own the copyright in the story and the characters. Spielberg (or, more accurately, his production company) got the copyright in the film and anything "new" in it (the visuals, the sounds, the lawyer getting eaten in the outhouse, etc).
So when someone makes toys based on the film, whose copyright covers that? That would depend on the terms of the contract between Crichton and Spielberg. By default, because Crichton is the original author of Jurassic Park, he must authorize every derivative work. Spielberg only has the right to create or authorize derivative works if Crichton sold that right in the contract. (These issues are one reason why California has a huge population of "entertainment lawyers.")
Unauthorized derivative works get no copyright protection at all. An illustration: screenwriter Timothy Anderson wrote a treatment for Rocky IV shortly after the release of Rocky III, and took it to MGM. Sylvester Stallone then wrote his own script, which was eventually used for the movie. Anderson sued, claiming that Stallone stole his story idea. But when the case went to court, Anderson lost. The court reasoned that because Stallone had written all of the previous Rocky scripts, and because Anderson had not received permission from Stallone before writing his treatment, the story was uncopyrightable, and Stallone therefore could not be infringing it. You can't steal what's rightfully yours. So the logic goes.
In keeping with the statute, derivative works must also have some element of "originality" to get their own copyright protection. One case arose when an artist drew a commemorative plate based on a scene from the The Wizard of Oz as a contest entry. The Bradford Exchange offered her a contract, but she refused it. Another artist copied the plate design, and the original artist sued for infringement. She lost because her design was deemed to be not original enough to merit its own copyright protection: only a couple of minor elements were changed from the original movie scene.
Sometimes, the distinction between derivative and non-derivative works is hard to discern. Reviews, for instance, are usually not derivative works, but episode guides and trivia books are. But while Paramount Pictures is likely to sue someone who publishes an unauthorized Star Trek episode guide in book form, they aren't likely to stop someone who does the same thing on a web site. Figure that one out...