It is not an infringement of copyright, however, to use short quotations from a work for the purposes of criticism, comment, teaching, scholarship or research. Any quotations used must clearly identify the name of the author and the source of the quotation. Note, however, that if your use is not fair use, then merely giving credit to the author of the work won't protect you. Although the above issues relate equally to the online world as they do offline, there are some additional issues that webmasters need to think about in the copyright arena. Firstly, just because you can download something for free doesn't, of itself, mean you can redistribute it freely.
Clip art is a good example.
There are numerous sites that offer free clip art. These sites allow you to download some of their clip art images free of charge. You can use these images for your own purposes in developing your webpages, for example, but just because they're free does NOT mean you can then turn around and make the images available to someone else for free or otherwise. Under these sorts of download arrangements, you're given a restricted license only. The same goes for free e-books.