A copyright is a type of intellectual property that protects the original works of authorship as soon as the author fixes the work into a tangible form. So, when you are using someone else's words, be sure to cite them correctly so that they receive credit for their original idea, and you do not run the risk of plagiarism.
Fair use is a US copyright law that states that brief excerpts of copyrighted material may, under certain circumstances, be quoted for purposes of criticism, news reporting, teaching, and research, without the need to ask permission from or make a payment to the original copyright owner.