Glad you asked...
A copyright protects your rights to a piece of work. Once you have the copyright, others cannot reproduce, perform, display, distribute copies of, or attempt to create their own works based on yours. While you do not have to obtain a copyright through the U.S. Copyright Office, it is beneficial to, since doing so will make it a matter of public record and you will have a better leg to stand on if someone infringes on your rights. You can find more information here:
http://www.copyright.gov/...
