What is Fair Use?
"Fair use is a legal doctrine that promotes freedom of expression by permitting the unlicensed use of copyright-protected works in certain circumstances. Secion 107 of the Copyright Act provides the statutory framework for determining whether something is a fair use and identifies certain types of uses—such as criticism, comment, news reporting, teaching, scholarship, and research—as examples of activities that may qualify as fair use."
Taken from "More Information on Fair Use" at Copyright.gov.
How Does Fair Use Work?
Generally, fair use allows the reproduction of certain works for the purposes of criticism, comment, news reporting, teaching, scholarship, or research, so long as the use falls within certain parameters. Fair use is not the same for every work in every situation, but certain factors can be weighed against each other to see if your use of a work falls under fair use. In determining whether the use made of a work in any particular case is a fair use, consider:
The fact that a work is unpublished shall not itself bar a finding of fair use if such finding is made upon consideration of all the above factors.
What does that mean?
Ask yourself these four questions before you use a work, and look up what best practices are when it comes to your field. Weigh these factors against each other, don't just assume one factor is enough to give you permission to use a created work however you want to. For example, according to the first factor, it may be okay to use a journal article for educational purposes in a class, but the third factor means it wouldn't be okay to post that article online for everyone to see even if you are using it to teach a class.