CloudSecurity blocks dangerous websites before they can cause harm to your PC or steal your data. And the more people who use CloudSecurity, the better it gets. Users can report suspicious websites via the plug-in back to G Data, whose experts then check over the websites to see if they are dangerous or not. If they are, they get added to the black list.
If you are currently availing of some of the free AV solutions out, then this sounds like a useful complementary (and complimentary) product.
(This type of product seems to be flavour of the month as Todd also mentioned a similar product in GNC #652 last week – Web of Trust.)