The two types of materials you will find on this page are public domain materials and royalty-free materials:
Public Domain - Public Domain materials have no copyright. They might be public domain because their copyright has expired (most pre-1923 materials) or because they are statutorily exempted from copyright, like most, but not all material produced by federal agencies. For more information on the status of state and federal government materials, please see http://freegovinfo.info/copyright.
Some post-1923 creators have explicitly dedicated their works to the public domain, waiving their rights under copyright law. An example of this can be found at http://www.archive.org/details/DeclanCory. You may do whatever you please with public domain materials and you do not have to pay for the usage of the work. Additional information about when an item becomes public domain can be found at http://copyright.cornell.edu/resources/publicdomain.cfm.
One popular family of licenses for royalty free works is Creative Commons CC. There are several types of CC licenses – some allow you to use the work in any way as long as you give attribution to the creator. Others will only allow you to use the work “as is” with no alterations. Yet other licenses grant different rights depending on whether your work product is a commercial use (i.e. whether it is given away or sold). Please review a work’s CC license before using it. More information about Creative Commons licenses can be found at http://creativecommons.org/license/.