For an introduction, before 1989 a published work had to contain a valid copyright notice in order to claim the protection of the copyright law. Since 1989 all works, whether or not the contain a notice are copyrighted automatically.
In reading genealogy blogs, I find that some bloggers tend to incorporate whole articles into their blogs. Some copyrighted material can be incorporated (quoted verbatim) without violating the copyright law. This limited incorporation is called Fair Use. Generally, fair use is copying of copyrighted material for a limited and/or transforming purpose. There have been hundreds and perhaps thousands of lawsuits filed over the issue of fair use, however, there is no dispute that incorporation of substantially all of a copyrighted work violates the copyright law.
Generally, copying a few selected paragraphs or illustrations for review or comment is allowed. In every case attribution (a citation to the source) should be included. Incorporating an entire poem, a copyrighted photograph or other copy of an entire work is an obvious copyright violation. Just because you have never had trouble in the past, does not mean that you will escape liability in the future. If you are challenged, the only way the dispute can be resolved in in the courts and the Federal Courts have exclusive jurisdiction over copyright cases.
As a note, works produced by or through the U.S. Government are not normally subject to copyright.
There are four limitations on the implementation of the Fair Use defense to a copyright claim. These four factors are set forth in the U.S. Code, Title 17, Chapter 1, Section 107:
Notwithstanding the provisions of sections 106 and 106A, the fair use of a copyrighted work, including such use by reproduction in copies or phonorecords or by any other means specified by that section, for purposes such as criticism, comment, news reporting, teaching (including multiple copies for classroom use), scholarship, or research, is not an infringement of copyright. In determining whether the use made of a work in any particular case is a fair use the factors to be considered shall include—(1) the purpose and character of the use, including whether such use is of a commercial nature or is for nonprofit educational purposes;(3) the amount and substantiality of the portion used in relation to the copyrighted work as a whole; andThe fact that a work is unpublished shall not itself bar a finding of fair use if such finding is made upon consideration of all the above factors.
Think before you use someone else's work.