Well, from what I have read you can use it to watch YouTube, Netflix, Flicker and iTunes content. Apart from that you could also connect your iPod/iPhone/iPad to it and play their content on your TV via the Apple TV box. But that is about all you can do with the Apple TV box. It is not a very versatile media player, depending on the model you have it may have a very limited storage capacity for media (8GB).
The Apple TV system is basically designed to play streaming media from the net or media stored on your Apple branded portable device.