You might be wondering why I've copied and pasted an image of a UFO here from OpenClipArt.org. This image is free to be used, remixed, and shared because it is licensed under Creative Commons. Creative Commons licenses helps content creators to share their work more freely than copyright allows. Because the UFO is a Creative Commons image, it is acceptable to edit and remix it to create and share new images (note that it contains no restrictions on use).
Copyright law protects a work the moment it is put into a fixed form (so the moment the words are written, the video is recorded, or a picture is snapped) and states that there are certain rights with regards to the work that only the creator holds. For example only the creator may reproduce the work (but see fair use and the other copyright exceptions, like face-to-face teaching). Fortunately, some authors and creators are happy and willing to share their work more freely than U.S. copyright law currently allows. In order to make their wishes clear to both you and the law, they often license their work. A license details the terms and conditions the author has established with regards to using his or her works. There are many different types of licenses but some of the most common and useful in an academic setting are Creative Commons Licenses.
If you are considering licensing works that you create under Creative Commons, be sure to browse through CreativeCommons.org's FAQ page. This page addresses questions regarding international rights, dual licensing, print works, and more. Once you have decided that a Creative Commons license is right for you, adding one to your work is actually very simple. You simply have to decide what rights to your work you would like to grant to users. Do you want them to be able to remix your work? Share it? Use it commercially? CreativeCommons.org provides a license chooser tool that walks you through these decisions. For more information about each, look through the About the Licenses page on CreativeCommons.org.