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 downloaded 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. Some examples of public domain sources:
Public Domain Sources
|Publications of the U.S. Government|
|Copyright has been waived by the author||All Free Original Clip Art (1)|
|Works on which the copyright has expired||Works by William Shakespeare|
1 Cartoons royalty free, provided by All Free Original Clip Art