According to the Library of Congress, "copyright refers to the author's (creators of all sorts such as writers, photographers, artists, film producers, composers, and programmers) exclusive right to reproduce, prepare derivative works, distribute copies, and publicly perform and display their works."
Almost everything is copyrighted and requires permission from its author to use. There are, however, some exceptions:
"Specially-licensed" - There is a lot of media that has been made available for anyone to use, waiving the need to obtain direct permission from the author. For example, a very common license is an "Attribution" license, meaning that you can use the media for any purpose as long as you give credit to the creator somewhere in your work. The non-profit organization Creative Commons offers a variety of attribution licenses that creators apply to their work to make available for public use. Learn more about Creative Commons licensing. Search for works with a Creative Commons license.
"Public Domain" - Works that are not copyrighted and are publically available are considered in the "public domain." Works exist in the public domain usually for one of three reasons: 1) they are very old, 2) the copyright has expired, 3) the work was created by the federal government. It can be hard to determine if something is in the public domain, so your best bet is to try to find a licensed alternative. Learn more about when different works entered the public domain. Find out if the work you want to use is copyrighted.
"Fair Use" is a provision in US copyright law that imposes limits and exceptions to the exclusive rights of authors / creators.
It means that you can use copyright-protected media without asking for permission if is considered fair, such as criticism, comment, news reporting, teaching, scholarship, and research.