Copyright insures that the person who created something--whether a book or a piece of music--is reimbursed for his intellectual work. If there were no copyright protection, there would be no economic incentive to create these works.
A copyright is a set of legal rights that an author has over his work for a limited period of time. Copyright covers everything from using images or sound files from the Web to photocopying. Most information is protected by copyright. Copyright protects the following eight categories of works:
The exception is work that is in the public domain, which can be reproduced or used by anyone. However, you still must credit the author. There are four common ways that works arrive in the public domain:
Public domain works are not protected by copyright. The public owns these works, not an individual author or artist. Anyone can use a public domain work without obtaining permission, but no one can ever own it.
Open access is free availability on the public internet, permitting any users to read, download, copy, distribute, print, search, or link to the full texts of these articles, crawl them for indexing, pass them as data to software, or use them for any other lawful purpose, without financial, legal, or technical barriers. The only constraint on reproduction and distribution, and the only role for copyright in this domain, should be to give authors control over the integrity of their work and the right to be properly acknowledged and cited. (Budapest Open Access Initiative)