Given that nearly all creative and intellectual output that has been created since the mid-1920s is protected by copyright, how can you use copyrighted materials in your own work? If you want to use music, clips of video, photographs, etc in your video, how can you do so and stay on the right side of the law? There are basically two ways: get permission to use the material or rely on "Fair Use".
Fair use is the part of the copyright law that ensures that copyright doesn't stand in the way of free speech, education, and other beneficial uses. Fair use is a four part balancing test that asks you to look at your planned use to evaluate whether or not your use is fair. Uses like parody, commentary, criticism, education, and news reporting are seen to be so beneficial that copyright shouldn't stand in the way.
For more information about the four factors of fair use and how to conduct a fair use analysis, please see the Copyright Resources Guide: Fair Use.
The Center for Media and Social Impact has done some great work over the last decade developing best practices documents for various "use communities". The idea here is to get people together who do the same thing (make documentary films, work with online video, teach poetry, etc) and figure out what the most common fair use situations are and provide some guidance about what is likely to be fair (or not fair) in those particular situations. Check out:
With a few exceptions, if your use is not fair, you need to get permission from the copyright holder or use materials that have already been licensed for your particular use. For more information about getting permission, see Copyright Resources Guide: Permissions
Creative Commons licenses are a way (though not the only way) for owners of copyrighted material to approve certain types of uses of their work without permission. Creative Commons licensed materials (except those explicitly licensed with a "Public Domain" license) are still copyrighted and the creator of the work still owns that copyright.
The purpose of this licensing is to offer a flexible and efficient mechanism for content creators to allow their materials to be used in certain ways without the burden of the permissions process.
Typically, CC licenses require attribution (saying who created it) at a minimum. Copyright holders can choose licenses that allow (or prohibit) commercial uses, declare whether adapting the work is permitted, and to require the user to also use a CC-license on derivative works.
More resources for finding copyright-free or licensed materials: