Geeks have long dreamed of digital home entertainment centers, their music, movies, and more all available at the touch of a button, without the mess of CDs, DVDs, and VHS tapes. But the technology hasn't worked well, despite a brief experiment by PC vendors with home media PCs, the rise of network-attached storage, and utilities like Plex. The many pieces didn't really fit.
That's changed, thanks to Apple. You can now put together a digital media center with the $99 Apple TV at its heart, iTunes as the engine that powers your media, and AirPlay as the protocol that brings mobile devices into the action.
P.S. For a great guide to AirPlay and Apple TV, check out Ben Harvell's book "iConnected."