In some cases an author will have gone through the process to have a novel's copyright in their name through the Library of Congress. When they sign a contract with a publisher (which is a licensing of rights), the publisher will hold all the rights granted in the contract.
If you sell/license North American Rights, you can't do anything with the material in the book in the North American market.
I shared my audio/video rights with my small publisher, so we were both free to market the book that way.
An agent (if you are represented) could try and sell publishing rights in other countries.
There is a Dummies book that explains the clauses in a standard book publishing contract, but the one I looked at twelve years ago would be really out of date. Some clauses are boiler plate, others are critical.
If you are offered a publishing contract, you should have it reviewed by someone who understands how the process works.