Fair use is a legal doctrine that promotes freedom of expression by permitting the unlicensed use of copyright-protected works in certain circumstances.
Source: U.S. Copyright Office
Determining Fair Use is done on a case-by-case basis each time you wish to use someone else's copyrighted work. To determine if your proposed use of a work is considered Fair Use, you must thoroughly review a these Four Factors:
Lengthier explanations and examples for all of these can be found on Copyright.gov from the U.S. Copyright Office.
1. Purpose & Character of Use - will your use be educational in nature, non-profit, for scholarly criticism or research? Or do you plan to sell or profit from using or embedding the work into your own? Will you significantly transform the original and add to it, or will you use it exactly as is? Will you create a parody of the original?
* Fair Use favors educational and non-profit use, as well as creative transformations.
2. Nature of the Copyrighted Work - is the item factual or mere reporting in nature, or is it creative? Is it published or unpublished? Remember, copyright law in the U.S. was intended to protect authors' rights and creativity, and to encourage publications.
* Fair Use favors use of "cut and dry" factual material and published material. (Using creative literary works, original research, music, films, photographs, and so on tends to lean away from Fair Use, as does use of unpublished works.)
3. Amount & Substantiality of the Portion Used - how much do you intend to use? A small part, or the entire thing? Or the most important parts?
* Fair Use favors small portions, "small" being defined in relation to the entire work. (However, if a "small" portion is the most important part of an item, that may not be considered Fair Use.)
4. Effect on Potential Market - will your use have little to no impact on the use or value of the original? Or will it have a negative impact, such as obscuring whose original work that was, or depriving the creator/copyright holder of sales or opportunity to distribute their own work? Will you use it just once, or repeatedly? According to copyright lawyers, this factor can also transcend the individual item and rather consider a scenario of "what would happen if everyone were to do this with this type of material."
* Fair Use favors no adverse impact on the original creator or copyright holder, or negative impact on market value of this type of material & use in general.