Excerpts from the United States Copyright Office on Fair Use:

One of the rights accorded to the owner of copyright is the right to reproduce or to
authorize others to reproduce the work in copies.

Various purposes for which the reproduction of a particular work may be considered fair
include: criticism, comment, news reporting, teaching, scholarship, and research.

Four factors to be considered in determining whether or not a particular use is fair:

1. The purpose and character of the use, including whether such use is of commercial nature or is for nonprofit educational purposes.

2. The nature of the copyrighted work.

3. The amount and substantiality of the portion used in relation to the copyrighted work as a whole.

4. The effect of the use upon the potential market for, or value of, the copyrighted work.

The distinction between what is fair use and what is infringement in a particular case will
not always be clear or easily defined. There is no specific number of words, lines, or notes
that may safely be taken without permission. Acknowledging the source of the copyrighted
material does not substitute for obtaining permission.

Examples of activities that courts have regarded as fair use:
  • quotation of excerpts in a review or criticism for purposes of illustration or comment;
  • quotation of short passages in a scholarly or technical work, for illustration or clarification of the author’s observations;
  • use in a parody of some of the content of the work parodied;
  • summary of an address or article, with brief quotations, in a news report;
  • reproduction by a library of a portion of a work to replace part of a damaged copy;
  • reproduction by a teacher or student of a small part of a work to illustrate a lesson;
  • reproduction of a work in legislative or judicial proceedings or reports;
  • incidental and fortuitous reproduction, in a newsreel or broadcast, of a work locatedin the scene of an event being reported.”

Copyright protects the particular way authors have expressed themselves. It does not
extend to any ideas, systems, or factual information conveyed in a work.

The safest course is to get permission from the copyright owner before using copyrighted
material. The Copyright Office cannot give this permission.

When it is impracticable to obtain permission, you should consider avoiding the use of
copyrighted material unless you are confident that the doctrine of fair use would apply to
the situation.

For more information: http://www.copyright.gov/fls/fl102.html