Fair use is a part of copyright law that recognizes certain uses of copyrighted works benefit society without causing undue harm to copyright holders and therefore do not require permission from the copyright holder.
Some of the examples listed as fair use in copyright law include criticism, commentary, parody, news reporting, teaching, scholarship, and research. However, not all uses for these purposes will automatically be fair.
Fair use is purposefully vague so that it can remain flexible as technologies and types of uses change over time. However, its vagueness can make it more difficult to decide when your use is or is not fair. In order to decide if a use is fair, you have to analyze the particular details of your situation and proposed use.
Everyone is free to exercise the right to make a fair use of a copyrighted work, but copyright owners are also free to disagree with a user's fair use analysis and to subsequently sue for copyright infringement. That means that those who want to exercise their fair use right will need to do so with careful thought and will need to weigh the risks involved in certain uses.
For examples of cases that courts have decided as either fair use or not fair use (copyright infringement), see: