Copyright is a set of legal rights that an author has over his or her 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. 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.
from "Bound by Law" by Keith Aoki, James Boyle, and Jennifer Jenkins of the Duke Center for the Study of the Public Domain. (This book is available under a Creative Commons Attribution-NonCommercial-ShareAlike license.)
If you want to learn more about copyright, check out these online resources.