Fair use is a part of the US Copyright Law (U.S.C. 17 §107) that sets forth limitations on the exclusive rights of copyright holders.
This means that under certain circumstances you do not have to seek permission or pay a fee to use copyright materials.
There are four factors to be considered when determining whether a particular use is fair or not.
A fair use evaluation must be actively conducted by weighing the four fair use factors for each individual case.
The purpose and character of the use
These types of uses are favored under fair use. Uses of a commercial nature usually would not be considered fair.
The nature of the work.
It is easier to show fair use with factual works than with creative works. Use of unpublished works is unlikely to be seen as fair.
The amount and substantiality of the portion being used.
The less used the better under fair use. Usage of key or main concept portions is often declared unfair use.
The effect of the use on the potential market for or value of the work.
Depriving a copyright holder of income usually weighs against a finding of fair use.