A copyright is a form of protection provided by the laws of the United States to authors of "original works of authorship." This includes literary, dramatic, musical, artistic and certain other intellectual works. This protection is available to both published and unpublished works. Material not protected by copyright (or otherwise protected) is available for use by anyone without the author's consent. An author of a copyrighted work can prevent others from copying, performing or otherwise using the work without the author's consent.
The copyright education center provides all the resources you need to understand various types of copyrights and the implications of not getting one. The FAQs should answer most of your questions.
Using LegalZoom, you can register a copyright at your convenience simply by answering a few questions online and sending in a copy of your work. We will even file the applications for you with the appropriate government agency.