What is Copyright
The purpose of Copyright is "to promote the progress of science and useful arts, by securing for limited times to authors and inventors the exclusive right to their respective writings and discoveries." (U.S. Const, art. I §8 cl.8) (more).
What Works are Protected
To qualify for copyright protection, a work must be original, and it must be fixed in a tangible medium of expression (more). Unpublished works may also be protected by copyright (more). Works which are not original, or which are not tangibly fixed, are not protected (more).
What are the Rights of Authors
Authors and other creators of protected works have the right to reproduce, distribute, create derivative works, public performance, and public display (more). Authors retain their rights for the duration of protection. The length of copyright protection is normally a function of the date of publication, the date of creation and the death of the author (more).
How Can I Use Other's Works
There are a number of instances where a user may infringe on an owner's rights. One of which is Fair Use, an affirmative defense. Fair use allows for the use of copyrighted material without permission under certain circumstances (more). Also, when the appropriate amount of time passes the work moves into Public Domain. Public Domain (often abbreviated, "PD") is a body of intellectual material which is not under copyright and may be used without permission (more). It is also possible for an author to make their work more usable by placing them under a Creative Commons license (more).