From medieval Europe through the last two centuries of American history, authors, creators and publishers have sought a legal mechanism to control their works and, by extension, protect their livelihoods. This legal mechanism is called "copyright." Copyright is defined as the exclusive right of a creator to reproduce, prepare derivative works, distribute, perform, display, sell, lend or rent their creations.
United States Copyright Law
Copyright is a form of protection provided by the laws of the United States (title 17, U. S. Code) to the authors of “original works of authorship,” including literary, dramatic, musical, artistic, and certain other intellectual works. This protection is available to both published and unpublished works. Section 106 of the 1976 Copyright Act generally gives the owner of copyright the exclusive right to do and to authorize others to do the following:
• To reproduce the work in copies or phoneorecords;
• To prepare derivative works based upon the work;
• To distribute copies or phonorecords of the work to the public by sale or
other transfer of ownership, or by rental, lease, or lending;
• To perform the work publicly, in the case of literary, musical, dramatic, and
choreographic works, pantomimes, and motion pictures and other audiovisual
works;
• To display the work publicly, in the case of literary, musical, dramatic, and
choreographic works, pantomimes, and pictorial, graphic, or sculptural
works, including the individual images of a motion picture or other audiovisual
work; and
• In the case of sound recordings,* to perform the work publicly by means of
a digital audio transmission.
*Note: Sound recordings are defined in the law as “works that result from the
fixation of a series of musical, spoken, or other sounds, but not including the
sounds accompanying a motion picture or other audiovisual work.”
The complete copyright liability handbook for librarians and educators by 
Copyright and E-Learning by 
Copyright handbook : how to protect & use written works by 
Copyright in cyberspace 2 : questions and answers for librarians by 
Copyright law and the distance education classroom by 
Technology and copyright law : a guidebook for the library, research, and teaching professions by Works in the public domain are not protected by copyright so you are free to use them. See the links below for guidance in determining if the work you want to use is in the public domain:
Creative Commons Licenses offer an alternative model to Copyright Law and allow authors to choose how their work can be reused. There are 6 licenses available. Here are examples of 2 of them:
CC BY - Attribution (least restrictive) lets other distribute, remix, tweak and build upon, commercial or non-commercial as long as the creator is credited.
CC BY-NC-ND – Attribution-NonCommercial-NoDerivs (most restrictive), allows others to download and share as long as creator is credited; can’t change them or use commercially