We have written about copyrights and fair use before. In reference to an intellectual property dispute that involved recording artist Taylor Swift, it was inferred that fair use would likely be the reason the case against her wouldn't progress very far.
Copyrights are a legal protection that govern original works by content creators. Literary works, musical pieces, paintings, photos and films are just some of the things that can be protected by a copyright. The copyright then forbids other people from repurposing or reproducing that work, unless the owner of the copyright allows it.
Now having said that, copyrights are not perfect. They do not bar people from using work protected by the copyright in all circumstances. This is where public domain and fair use come in to play. If content enters public domain (either by qualifying due to being a part of a certain era of time, or by permission from the content creator) then it is no longer protected by copyright law. Additionally, if someone uses copyrighted content under "fair use," then it, too, is not protected by copyright law.
Fair use is a little more complicated. If someone uses content that is copyrighted under certain circumstances -- such as using it to criticize or comment about a topic, using as part of news, using it to teach or using it to make a parody of the content -- then it falls under "fair use" and the copyright holder can not make a claim against the person using their content.