The Creative Commons project is a non-profit community where users allow use, sharing, and remixing of their original content at no charge to the public. The content creators can set limits to the use of their material (audio, video, photographs, literature, etc.). Many websites, including Wikipedia, will license their content under Creative Commons to encourage use and sharing.
There are literally millions of Creative Commons-licensed materials available online. The aim of contributors and users is not necessarily to circumvent copyright, but to participate in a "copyleft" movement that seeks to create a richer body of public domain works. For the purposes of academic research, creative commons material is a valid, legal, and fair source of media for presentations and projects.
An easy way to discover Creative Commons-licensed materials is through the Creative Commons Search page. You can search for images, videos, music, web pages, and media directly from this page.
To help you easily identify licenses, Creative Commons uses icons to identify the type of restrictions, or lack thereof, for a work. This is not an exhaustive list, but contains the most frequently used icons.
Attribution (BY) - give credit, or attribution, to the creator of the work
Non-Commercial (NC) - work cannot be used in a commercial work unless additional consent is obtained
No Derivatives (ND) - use the work as is and do not edit or manipulate work and distribute it
Share Alike (SA) - release the new, or derivative, work under the same license type
Public Domain (CC0) - freely build upon, edit, share, and transform the work without restriction
No Known Copyright (PDM) - freely build upon, edit, share, and transform the work without restriction; reserved for older works whose copyright expired
You may also see icons combined together to form buttons. Here are two examples of Creative Commons license buttons.
Take a look at this blog posting for a much more thorough discussion of how these Creative Commons elements fit together:
Watch this cute and fun five minute video about Creative Commons to learn more.