Google Talk has been around for quite a while. Most people saw it used in Gmail and it was added to Google+ as well. Even on the new social network, the chat widget has remained unchanged until now.
Google is making a subtle but very important change to the Google chat box, you can now talk to anyone in your Circles, not just the people you've added manually or had conversations with in Gmail.
"Google Chat makes it easy to chat with people you have relationships with. In Google+, you can chat with anyone in your Circles who also has added you to their Circles," Google explains
"And, when you're not using Google Plus, you'll still be able to chat with them using Google products including, Gmail, the Google Talk client, iGoogle, Orkut and 3rd party apps," it adds.
It's an obvious move and one that should get a lot more people using the chat box in Google+. That said it's probably going to annoy at least some people that were already using the chat feature in Gmail and other places.
Google has been working on integrating Google+ in all of its products. Adding support for Circles in Google Talk is a logical step, maintaining two separate lists of friends/contacts doesn't really make sense, from Google's point of view.
With the new chat feature, you can talk to anyone on Google+ that you have in your circles and that has added you to theirs. Of course, existing contacts will also be available, even if they're not on Google+ and will sit on top of the friends list.
In fact, most Google+ friends won't show up in the chat list unless you've already chatted with them recently. You can look them up in the search box though.