No, there was no intro for that ... again, it is something new, and we may remove it if people don't find it useful. Use it if you want, otherwise just ignore it!
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They are just bookmarks like you have in your browser. But these four are for popular sites, and are the defaults the software suggests. For example, one can become a member of "Digg" and submit a story from somewhere on the Internet - like Pet Talk - that one thinks might be of interest to the "Digg.com" audience.
deli.icio.us and StumbleUpon are similar - we have had people find our site because someone else 'stumbled" it, and the more people do so, the better. Google is, well - just Google.
Again, these are just features of the latest upgrade of the vBulletin software. We don't need to keep every single new feature, but figured we might as well at least try them before we dismiss anything new out of hand.