Under copyright law, a joint work is a work created by two or more people who share equal interests in the finished product. In order to be considered a joint work, the contributions of all of the authors must be interdependent and inseparable. Thus, if someone writes a forward to a book written by someone else, this is not a joint work, but if a writer and illustrator collaborate to make a children's book, for example, they would be considered joint creators. Works for hire can also be considered joint works, with the company or person that commissions the work having an equal interest in the copyright of the work.
Books, journal articles, musical compositions, and works of art can all be joint works. The size of a contribution does not matter. If someone contributed to a joint work, that person shares an equal part of the copyright and has equal rights. The creators may establish a contract to clarify certain points of their collaboration.
A joint author has the right to assign nonexclusive rights to another party without consulting the other authors. Likewise, joint authors can transfer their interest in the work to someone else, as when someone wills a share in a copyrighted work to heirs. Parties to a joint work cannot assign exclusive rights without the consent of the other authors. They must also account for any profits they earn by exploiting the work.
The contributions to a joint work are meant to be viewed as part of a seamless and interconnected whole. Without the work of one of the contributors, the piece would not be finished. Authors may choose to explicitly identify their contributions to the work, as when authors write alternating chapters of a book, but they are not required to. The contributors are usually named, but in the case of people like ghostwriters, a contributor's name may be publicly left off the finished joint work.
There are situations where it may not be to someone's advantage to have a finished product classified as a joint work. Some people like to be able to control all of the ways that their work is used and they may not like the idea that other contributors can assign non-exclusive copyrights without consent. Likewise, if conflict arises between the creators, it may be difficult to reach agreements about issues pertaining to the work that do require input from all the authors.