The idea of fair use is complicated and, at best, vague. In general, the
'fair use' clause of copyright law was added so that people can use small
portions of copyrighted material for commentary, education, and criticism.
Please note that fair use does not mean you can post copyrighted materials as
long as you are not making some kind of profit from them. That commonly held
notion is false. Also note, that in commentary, education, and criticism, the
entire document is usually not replicated, only small portions are quoted.
Always keep in mind that the original author has the right to make copies of,
to sell, and to distribute his or her work. If you are commenting on a book, for
example, and you duplicate the entire manuscript, even if you are doing so for
commentary purposes, you are making that work easily accessible to the public,
which means the original author may suffer profit loss.
What to Remember
If you are ever in doubt as to whether you are bordering on copyright
infringement, contact the source and ask for permission to use the excerpt that
you want to use. Always use proper quotes and citations, and be original. If
you use your own ideas, you can be sure to avoid possible complications.