Fair use is a right that recognizes that there are certain uses of copyrighted works that benefit society without causing undue harm to copyright holders and therefore do not require permission from the copyright holder.
Some of the "classic" examples of situations where a use is likely to be fair 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 their fair use rights, but in most cases, only a judge can definitively decide if a use is fair or not when a copyright infringement suit is brought to court. 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.