Copyright, literally, is “the right to copy.” It guarantees the authors of creative works–including books, artworks, films, recordings, and photographs–the exclusive right for a set period of time to allow other people to copy and distribute the work, by whatever means and in whatever media currently exist. It also prohibits copying and distributing without the author’s permission.
Contained within copyright is the entire bundle of rights that an author can grant to others or utilize him/herself.
When you sign a publishing contract, you are granting the publisher permission to exploit (i.e., to publish and distribute for profit) some or all of your rights for a defined period of time. Because you own thecopyright, granting rights doesn’t mean you lose or abandon those rights–merely that you authorize someone else to use them for a while, either exclusively (i.e., no one else can use them at the same time) or nonexclusively (i.e., you can also grant them to others).