Copyright law limits the exclusive monopoly it grants authors in several ways. It grants exceptions for many of the reproduction activities typically performed by libraries and for performance and display uses typically practiced for in-person classroom instruction. If all of the code’s conditions are met, such specific uses are not copyright infringements.
Copyright law places a high value on educational uses. The Classroom Use Exemption (17 U.S.C. §110(1)) only applies in very limited situations, but where it does apply, it gives some pretty clear rights.
In order to determine whether your use applies to this exception, use the Exceptions for Instructors in U.S. Copyright Tool.
There are lots of other exceptions, exemptions, and limitations in copyright. A few interesting examples: