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Broadcast accounts are things like Twitter, Facebook or Google Buzz, which allow you to send updates and share things with a number of friends or followers over the internet. They allow you to keep up with whatever your friends are doing. Unlike email, broadcasting is more like a mail shot out to a group of people at the same time. It's not just text that can be shared, but files, images and video (if your connection's bandwidth allows)!
Linux has a number of tools to handle broadcast messages, but the default for Ubuntu is Gwibber. Gwibber allows you to define all your social media accounts in one place, then to broadcast out across all of them at the same time. As an example, the next section will show you how to configure Gwibber with a Google Buzz account (-but the priciple is the same for the likes of Flickr, Facebook, Twitter, etc).