Materials in the Public Domain. Works that are in the public domain are no longer, or never were, protected by copyright law. For example, anything created by the federal government is public domain -- everybody owns it! Also, any work published on or before December 31, 1922 is no longer protected by copyright and may be used freely. Some things, like data and facts, are simply not protected by copyright law. Public domain works are a great choice for your multimedia project, since you are free to use them in any way you wish. Read more about the public domain.
Materials with a Usable Creative Commons License. Creative Commons is a non-profit organization created to facilitate sharing content by creating an organized system of permissions. When creators mark their works with Creative Commons symbols, they provide ready-made licenses that tell other people exactly how the work can and cannot be used. You will see these symbols and licenses throughout the web. Be warned that there is a wide range of licensing and permissions -- do not assume that all licenses are the same. Some users may allow you to use their images/music/video for non-profit use, but not for commercial projects. One requirement that applies to all Creative Commons licenses is that you must include an attribution to the creator of the original work in project. Read more about Creative Commons.
Materials with Permission from the Copyright Holder. Direct, written permission from the copyright holder is the ultimate safe bet. However, this can be difficult, time-consuming, or impossible to obtain, so it is wise to consider your other options first. Read more about requesting permission.