assuming that nothing posted here is of a commercial nature, the key concept is "fair use". here is the statutory language about fair use found in Section 107 of the Copyright Act:

Notwithstanding the provisions of section 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; 2) the nature of the copyrighted work; 3) the amount and substantiality of the portion used in relation to the copyrighted work as a whole; and 4) the effect of the use upon the potential market for or value of the copy righted work.

so there is the (excerpted) legalese. here is something I copped from somewhere: When you are about to make a copy, a strident annoying bell should go off in your head. You should heed this as a warning that you need to think clearly and objectively about what you are about to copy. Put yourself in the place of the person whose work you want to copy; how would she feel about what you want to do? [emphasis added]

http://www.efuse.com/Plan/copyright2.html