To make a reasonable judgment about whether the use you make of a copyrighted work is Fair Use, the Copyright law provides a set of four criteria in Sec. 107. These criteria include:
It is important to note that while no one factor is paramount, all four factors should be considered in your analysis. Note also that the preamble to Sec. 107, lists criticism, comment, news reporting, teaching (including multiple copies for classroom use), scholarship, or research as examples of Fair Use. That does not mean that ALL used for these purposes are necessarily Fair Use, but it does point to the purposes that Congress had in mind when writing Sec. 107.
In recent years, the courts have embraced a concept of "transformative use" as a measure of Fair Use. Stated simply, transformative use refers to a re-purposing of the work rather than simply reproducing it, often in a different context and for a different audience. Or, as Kevin Smith has noted in somewhat more economic terms "the new use does not compete with the original [use] in the same market." (Kevin Smith. Owning and Using Scholarship, 114).