If your use of materials satisfies the requirements of the fair use exception, your use does not violate copyright (even if the materials are protected by copyright). There is a four factor test (discussed on this page) used to determine whether fair use applies.
Fair use is not a bright line test -- the four factors are weighed, and balanced, to determine whether a use is "fair." Don't be afraid to apply fair use. It can be a valuable tool when making use of materials.
The Four Factors
The four factors to consider, when evaluating if a use qualifies as fair use are set out in 17 U.S.C. §107:
"the purpose and character of the use";
"the nature of the copyrighted work";
"the amount and substantiality of the portion used"; and
"the effect of the use upon the potential market for or value of" the work.1
No one of these factors are controlling, and the determination is based on a balancing of the factors.
A factor recently found to be supportive of fair use is when the use is considered a "transformative" use. A transformative use is a use which "adds something new, with a further purpose or different character, altering the first with new expression, meaning, or message." Campbell v. Acuff-Rose Music, Inc., 510 U.S. 569, 579 (1994).2
Other factors which, according to the statute, may support fair use are: "criticism, comment, news reporting, teaching . . . scholarship, or research." However, even if a use is for one of these purposes, that does not guarantee that the use will constitute a fair use. And a use can still qualify as "fair use" if it is for a purpose which is not listed in the statute.
There are fair use checklists which can help you decide whether the use you want to make of a work constitutes "fair use." See the resources box on this page for some fair use checklists.
A good tip for establishing fair use is to use as limited a portion of the material as you can to achieve your purpose. For example, instead of copying an entire page just copy the paragraph you need (assuming you only need the paragraph); or if you need to use an image, only use a smaller (thumbnail) instead of a full image, if that is sufficient to your purpose.
No set amount constitutes -- or doesn't constitute -- fair use. Using ten seconds of something might constitute fair use; but it might not. You have to evaluate all of the fair use factors in making the determination, not just the portion used.