Web Design in a Nutshell, 3rd Edition

External Links

By default, any link within a frame loads the new document into that same frame. To prevent external links from loading into the current frame, be sure to add target="_top" to all your external links; the new site will open in the full browser window. As an alternative, set the target to "_blank" to open the link in a new browser window.

Tip

As noted earlier, always provide a note or some indication that a link is going to open in a new window so as not to confuse your users.

It is never appropriate to load whole external sites into the context of another framed document, unless you are doing so with the expressed permission of the owners and operators of the external site.