The "Fair Use" clause in Section 107 of the Copyright Law protects instructors when using copyrighted works for instructional purposes in the physical classroom. The TEACH Act protects instructors in the virtual classroom.
Visit the Fair Use Check List to determine if your course materials are appropriate for delivery in the classroom.
Use the University of Texas's checklist to determine if your materials are appropriate for delivery in a web course.
Additional Copyright Resources
Copyright Basics (.pdf) -- U.S. Copyright Office
Crash Course in Copyright -- University of Texas System
Copyright and Fair Use -- Standford University
TEACH Act Toolkit -- North Carolina State University
What is Public Domain? -- University of California
What is CC? -- Creative Commons