From the official announcement: "Today we're excited to launch Hangouts On Air to Google+ users worldwide. So if you have something to say—as an aspiring artist, a global celebrity, or a concerned citizen—you can now go live in front of a global audience.
With just a few clicks, you’ll be able to:
Broadcast publicly. By checking "Enable Hangouts On Air," you can broadcast your live hangout—from the Google+ stream, your YouTube channel or your website—to the entire world.
See how many viewers you’ve got. During your broadcast, you can look inside the hangout to see how many people are watching live.
Record and re-share. Once you're off the air, we’ll upload a public recording to your YouTube channel, and to your original Google+ post. This way it's easy to share and discuss your broadcast after it's over.
Of course, launching millions of live stations takes some doing, so we're rolling out Hangouts On Air gradually, over the next few weeks."
Official announcement: http://googleblog.blogspot.com/2012/05/google-hangouts-on-air-broadcast-your.html
Find out more: http://www.google.com/+/learnmore/hangouts/onair.