The deuterocanonical books (sometimes collectively called the Apocrypha or apocryphal books) are texts considered canonical by some Christian traditions but not others. These books are included in some Bibles but omitted from others. Some Christians, particularly in the Protestant tradition, do not consider these books to be part of the biblical canon--that is, they don't consider them to be equal in authority to the other books of the Bible. The deuterocanonical books, thus, does not appear in most Protestant Bibles. Other Christian traditions, including the Roman Catholic church, do consider them canon.
For information on reading the apocryphal books on Bible Gateway (or toggling their display on or off in your Bible Gateway searches), see this article: http://support.biblegateway.com/entries/187535-are-the-deuterocanonical-books-apocrypha-included-in-the-bible-gateway.