Public Domain works are not protected by copyright law and are, therefore, freely available for everyone to use.  Works may not be protected by copyright for a number of reasons:
 

• Work does not qualify for copyright protection.  A work must be original, be "fixed in a tangible medium of expression," and have some degree of creativity in order for it to be protected by copyright.  Here are some examples of materials that do not qualify for copyright protection:
  • Ideas, common facts, hypotheses, theories
  • Names, short phrases
  • Discoveries, processes, systems
  • Caveat lector: although ideas, facts, discoveries, hypotheses, theories, processes and systems may not be protected by copyright law, any expression of them could be protected.  For instance, the prose describing a scientific hypothesis in a journal article would be protected by copyright law, but the facts and data surrounding the hypothesis would be in the public domain. 
     
• The work's duration of copyright expired or was not renewed.
  • Due to changes in copyright law and treaties, determining the duration of copyright on a work can be very tricky.  In more complicated cases, it may be necessary to consult an attorney or copyright expert. 
  • The tab, "Is it a Public Domain Work?" in this tutorial gives charts and links that could be of some assistance.
  • Works published in the United States prior to January 1st, 1928 are in the public domain. 

• Copyright owner dedicated the work to the public domain.  Yes, these creations do exist!  Check out these photos posted on Flickr.com tagged with a cc0--or public domain--license.
   
• Work was created by the U.S. Government.
  

What this means for your academic projects is that you can use as much of the work as you would like to support your instruction, research, publication, creative work, etc. without needing permission from the original copyright owner.  Here are some examples of what you can do with public domain works:

• You are an MFA candidate in Media Arts or Theatre and want to create your own adaptation, play script, or screenplay of Willa Cather's Pulitzer Prize-winning novel, One of Ours-- originally published in 1922. 
• You are an English Literatures student analyzing some poems written by American poet, Elinor Wylie, from the collection of poetry, Nets to Catch the Wind, published in 1921 and would need to reprint the entire poems in your project.  
• You are a Music student who would like to publicly perform Beethoven's Cello Sonatas in a recital. 
• You are a faculty member in Art publishing a book about World War II posters created by U.S. Government agencies and want to reprint them in your book. 
• You are a Ph.D candidate in one of the sciences and would like to challenge a scientific hypothesis made by another scientist for part of your dissertation.   

Creative Commons works are works that are still protected by copyright.  However, its authors have chosen to allow certain kinds of uses-- such as being able to copy the work or to share it with others-- without the need to seek permission through Creative Commons license agreements.  

The best description about the Creative Commons is--of course-- from its own web site, "What is CC?": http://creativecommons.org/about/what-is-cc

Here is a summary of the basic kinds of Creative Commons licenses and the uses that they allow.  These definitions are from the "Licenses" page from Creative Commons. 

1. Attribution (Abbreviation: BY): This license allows the most kinds of uses.  Users may copy, create derivative works, perform, distribute, or display the work without permission; however, users must attribute you as the author in the way that you desire.   

2. Non-Commercial (Abbreviation: NC): An author can use this license to ensure that others may use their work, but not make money from using it. 

3. No Derivative Works (Abbreviation: ND): This license will allow a user to copy, perform, distribute, or display a work, but they may not create a derivative work.  For instance a user may not create a movie based on a book covered under this kind of license. 

4. Share Alike (Abbreviation: SA): If you use a work with this kind of license, any derivative work you create must also be covered by the same license.

These four kinds of licenses can be mixed and matched to create a set of use conditions customized to the author's desires. 

* Public Domain (Abbreviation: CC0): It is also possible for an author to release their work into the public domain by using this kind of CC license. 

Creative Commons licensed works can apply to all formats of works. 

Disclaimer: This tutorial on using public domain materials, Creative Commons licensed materials, and copyright law is provided for informational purposes only!  I am not a lawyer and cannot provide legal advice.  None of what you read in this tutorial should be construed as legal advice.  Should you require legal advice, please contact an attorney.