Public Domain and Fair Use - What does it mean?

Definitions:

"Public domain" refers to works that are available for unrestricted copying by the general public without prior permission. Material that resides in the public domain includes works whose:
copyrights have expired
works that were created too early to have copyright protection (prior to 1978)
works by the federal government
works donated to the public by authors or artists
http://fairuse.stanford.edu/Copyright_and_Fair_Use_Overview/chapter8/index.html

"Fair use" allows a person to copy limited amounts of copyrighted material without requiring prior permission. Four factors must be evaluated to determine whether use is "fair" or not:
The purpose and character of the use (most importantly whether it is for commercial gain or for nonprofit educational purposes)
The nature of the copyrighted work (how creative or non-creative is the work)
The amount and substantiality of the portion used in relation to the work as a whole
The effect of the use upon the potential market for or value of the copyrighted work
http://fairuse.stanford.edu/Copyright_and_Fair_Use_Overview/chapter8/index.html

For more detailed information scroll down to the bottom of this page: http://peacefulmannlinks.blogspot.com/

Honestly I have never had a problem obtaining copywrite to use for schooling purposes and for sharing with other teachers.

A side note, share sites that are for educational purposes, require enrollment, are not for profit, are allowed to share certain copywrite material within limited amounts.