When starting a Hangout, you now have the ability to write in a "topic" to help people know what your hangout is about. You can change the topic at any point during the hangout as well, although, this will not update the original Google+ post.
Hangout Call Back
Google has built Hangouts to be more casual than typical video conferencing. You just start a hangout, invite some circles, sit back and see what happens. This approach has its pros and cons, of course. On one hand, there is a service like Skype, where you either pick up and start the chat, or you don't pick up, and that stops the affair dead in its tracks. On the other hand, you've got Hangouts, where you can invite a bunch of people, talk with some of them, have people wander in and out, and then just bail when you get bored. The downside here is that a person you invited may join the Hangout after you've already left. Enter: Hangout call back feature! Made for just such an occasion, the call back feature will call the person who started the Hangout if someone joins after they've left.