Fair use is a doctrine that only really exists in the law of the United States that permits the limited use of copyrighted material without having to first acquire permission from the copyright holder.
Examples of fair use in United States copyright law include commentary, search engines, criticism, parody, news reporting, research, and scholarship. Fair use provides for the legal, unlicensed citation or incorporation of copyrighted material in another author's work under a four-factor balancing test.
Fair use is one of the limitations to copyright intended to balance the interests of copyright holders with the public interest in the wider distribution and use of creative works by allowing certain limited uses that might otherwise be considered infringement.
So all of this begs the question of what is fair use in reality. It's clearlry complicated, and there is no obvious line that can be crossed. In its most general sense, a fair use is any copying of copyrighted material done for a limited and 'transformative' purpose, such as to comment upon, criticise, or parody a copyrighted work, according to Stanford University.
Such uses can be done without permission from the copyright owner. In other words, fair use is a defence against a claim of copyright infringement. If your use qualifies as a fair use, then it would not be considered an illegal infringement.
Fair use is also decided on a case-by-case basis.
"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," according to the US Copyright Office.