Fair use is built in to copyright law to allow limited use of copyright protected material for transformative purposes. There are no hard and fast rules regarding fair use, but in cases of dispute regarding fair use, there are four factors which courts consider, which you can use to help guide your decision making.
For more information, refer to this guide created by Richard Stim
Are you using the material for a purpose different than the original? Have you use the work to create some new expression? Have you added value to the original work through your own work?
Examples of transformative use involve educational uses, scholarship and research, criticism or commentary, parody, and news reporting.
Nature of the copyrighted work
Considerations for the nature of a work include its general availability and accessibility, and whether it is a creative work versus nonfiction. Additionally, unpublished works are more likely to be protected from fair use than material that has already been published.
The amount or substantiality of the work being used
Generally speaking, it is more acceptable to use small parts of a work than reproducing large portions. However, it is also not considered fair use to reproduce the "heart" of the work (for example the most memorable part of a film), even if it is relatively small in size in relation to the whole work.
The effect of use on the market or the value of the work
This factor takes into consideration the consequences use of a work could have on the copyright holder. This is linked to the other three factors. If you are reviewing a movie, and include several lines of dialog, you are not diminishing the market for the movie. However, if you purchase a textbook and distribute copies to your classmates or students, you are diminishing the market for that product.