The details seem a bit foggy to me, but it sounds like @anywhere takes many of the Twitter API functions/services and makes them available to web sites in a much simpler way: by adding a few lines of javascript. Some of the mentioned uses of this platform include:

• being able to sign-in to third party web sites with your Twitter ID
• a New York Times journalist’s byline would show her Twitter profile and let anyone follow her from the article on the Times’ web site
• being able to tweet about a video from its YouTube page
• discovering new Twitter users/account from Yahoo’s home page

Those are some of the uses listed in Danny’s liveblog and/or the Twitter blog post, both of which are linked above. DigitalBeat says the platform uses Twitter’s hovercards (pop-up windows) and may include advertising, too.

When @anywhere launches — and I haven’t seen any indication when that’ll be — there will be 13 partner sites, shown here:

I notice that Bing and Yahoo are there, but Google isn’t. Google’s YouTube is a partner, however.

There’s more discussion on Techmeme, too.

Postscript: Yahoo has sent us a statement that explains a little more about its participation in @anywhere. The statement reads, in part: “…we’re excited to support @anywhere, allowing our users to authorize Twitter data sharing with their Yahoo! ID in a way that empowers them to consume their Twitter feeds on Yahoo! and to share Yahoo! content to Twitter.”