Copyright gives authors and creators the power to allow or prohibit certain uses of their work. "The main goals of copyright are to encourage the development of culture, science and innovation, while providing a financial benefit to copyright holders for their works, and to facilitate access to knowledge and entertainment for the public." (Copyright Clearance Center)
Some copyright basics for authors:
As a copyright owner, some of your rights are to:
Copyright and Publishing Agreements
If you transfer your copyright to a publisher, you have basically given up your rights as the copyright owner. If you wish to reproduce the work or prepare derivative works from it, you will need to seek permission from the publisher.
It's important to note that it is not a requirement of publication that rights be assigned or transferred permanently to a publisher. See AUTHOR RIGHTS tab.
"It is exceptionally rare for a scientific publisher to use copyright law to defend the integrity of a scientific paper on behalf of an author. In fact BioMed Central knows of no situation where this has happened...
Meanwhile, the real reason for copyright transfer is clear. Publishers regularly use copyright law to protect the profits they derive by controlling access to the literature."
Image: By Joan M. Borràs (ebrenc), CC BY-SA 2.5