Many people view the Internet as a virtual library full of books, essays, articles, etc.
However, there is a problem with that viewpoint. The documents on the Internet, unlike the books in the library, are difficult to catalog since they are constantly in a state of flux. Many Internet resources can disappear with the touch of a Webmaster's delete key. You should save or print all documents that you cite so that you have not only have a backup in case the site disappears, but you will also have proof you have cited an authentic source. Much of the text on the Internet was originally published elsewhere, so you may have access to the original source in the library.