It's a service that lets you bookmark pages, add comments and add tags in a similar way to how Google Notebook works, but with an emphasis firmly on sharing. As well as a 'shared stuff' page, your friends can add your Shared Stuff as a Google Homepage module, or subscribe to your RSS feed.
Once you've 'shared' an item, Google will tell you how many times it's been shared (like every social bookmarking site ever) but also how many times it's been viewed.
It looks like they'll be building this in to all the Google services that allow sharing (Maps, Video, YouTube, Reader, Picasaweb, Blogger, ...), but at the moment it's only supported in Google Video and via a browser (Share This) button. This suggests this is the mysterious Mocka-Mocka social service suggested in the Google Reader training video that was leaked earlier this month.
It's still not properly launched,
As it is, it's a very workable replacement for sending emails of cool stuff you've found online (or having a blog for the same).
For those playing at home, this accounts for the mysterious Google service 'S2'.