Fair Use is an exception in the copyright law that allows small portions of a work to be used without permission for nonprofit, news, educational, or parody purposes.
The doctrine is however often misunderstood. To fall under "Fair Use", only a small portion of the work may be used and the use must not affect the value or marketability of the work.
Selling concert recordings or photocopying sheet music without permission would not fall under fair use.
Check out the following links for more information on Fair Use:
The official Copyright Office explanation of Fair Use.
Useful resource from YouTube about Fair Use.
An article covering Fair Use in detail.