Broadcast is a WordPress plugin for network / multisite installs that helps authors keep their content synced between blogs on the network.
The following video shows Broadcast in action, broadcasting a post to several other blogs / sites.
To use it, just click on a blog or two in the Broadcast meta box when editing a post, and publish the post normally.
The above screenshot shows the Broadcast meta box, after having broadcasted the post to the blog called Broadcast 2. When updating this post, the equivalent post on Broadcast 2 will be automatically updated.
The blogs listed are the ones the user is an actual member of, with some kind of write access (contributor, author, etc).
The post on this blog is called the parent post, and the equivalent post on Broadcast 2 is called the child post.
If you were to broadcast from Broadcast 2 to another blog, the post on Broadcast 2 would then be the parent post.
You can see the relations between posts in broadcast column in the post overview.