NOTE: TwitchDeck v2 is under development, and will supplant this version of TwitchDeck once released. Consider this version of TwitchDeck deprecated.
TwitchDeck is an app designed to let a user watch multiple Twitch.tv streams at once. Beyond that, though, the player includes automation features that let the user prioritize a list of streamers from most to least favorite; TwitchDeck will then display their favorite online streamer in a main video player with a chat panel next to it. Other online streamers are displayed in a panel (called the Deck) below, with volume muted. That way, users don't need to manually fiddle with controls when their streamers go online or offline; TwitchDeck will automate it for them.
Users can maintain their list of preferred streamers, as well as setting up preferences, by creating a free account. This will let them go straight to the TwitchDeck player without having to specify which streamers they'd like to watch.