Can I use someone else's copyrighted work?
Copyrighted work may be used with the copyright holder's consent or for certain purposes by people other than the copyright owner under the Fair Use doctrine. The Fair Use doctrine allows reproduction for specific and limited purposes such as criticism and comment, news reporting, teaching, scholarship, and research.
What is considered consent by the copyright holder?
Some copyright holders intend for others to use and distribute their work, and may give unlimited permission in their copyright statement itself. If this is the case, print off a copy of the holder's copyright statement to keep on hand. In any case, consent from a copyright holder must be obtained in writing before you can use that person's copyrighted work.
How consent is obtained
Sometimes you can directly ask the copyright holder to grant you permission to use the work through a written letter. If granted, the copyright holder's consent can sometimes be expressed using a custom-written copyright statement. In some circumstances, you may need to pay for permission to use the copyrighted work, but most of the time the copyright holder simply prefers to know your intended use before granting permission.
More on the Fair Use Doctrine
The Fair Use Doctrine is an exception to the copyright protection rules that allows people to use someone else's copyrighted work without permission for specific purposes. Under this exception, you may use someone else's work without permission for criticism and comment, new reporting, teaching, scholarship and research. If your use does not fall under any of these exceptions, then you must seek permission before using the work. Consequently, it is always in your best interest to still inform the copyright holder of your intended use, even it does fall under the Fair Use Doctrine.
This article provides a basic definition of the two ways that copyrighted work can be used by someone other than the original copyright holder. If you are planning to use someone else's copyrighted material and are unsure if your use is lawful, consult with an intellectual property attorney (this includes copyright and patent attorneys).