The copyright owner has the exclusive right to sell, transfer, lease, rent, distribute, perform, and display the work. If the copyright owner wants to transfer some or all of her exclusive rights in and to a work to another, she must sign a written agreement. An oral promise or alleged transfer of copyright is invalid. Although an oral transfer of a non-exclusive license may be done, it can easily be revoked and invalidated at any time. Therefore, non-exclusive transfers should also be executed in a writing.
What Gets Transferred
Copyrights are rarely sold or transferred in their totality. Any right not specifically sold or transferred in writing remains with the original copyright owner. Generally the sale of all or part of the copyright is evidenced by a written License Agreement and / or Copyright Assignment. A License or Copyright assignment may be broad or narrow. The copyright owner may grant an exclusive or non-exclusive right to use all or a portion of the copyrighted work and the copyright owner may limit the copyright license or copyright assignment in time, geography, language, type of use, method of distribution, etc...
The Non-Exclusive License
Under a non-exclusive license the copyright owner is retaining all of her rights in and to the copyright, but granting a third party permission to use all or a portion of her work in some manner. The copyright owner may grant a non-exclusive license to as many people as she deems appropriate. Since a non-exclusive license is not a transfer of copyright, the holder of a non-exclusive license cannot sue for infringement, only the owner of the copyright has that privilege.
In addition, if the copyright owner grants someone a non-exclusive license and then later sells the copyright to a third party, that third party cannot later revoke the non-exclusive license previously given, so long as the license was in writing.