You can place a hyperlink to a given URL (Universal
Resource Locator) using
the \xlink command. Like the \link command, it takes an
optional argument, which is typeset in the printed output only:
In the HTML-document, anchor will be an active hyperlink to the object URL. In the printed document, anchor will simply be typeset, followed by the optional argument, if present. A starred version\xlink{anchor}{URL}\xlink{anchor}[printed reference]{URL}
\xlink* has the same function as for \link.
If you need to use a ~ in the URL of an \xlink command, you have
to escape it as \~{} (the URL argument is an evaluated argument, so
that you can define macros for common URL's).