So what does it do exactly?
A Chromecast? It's basically a little flash drive you plug into your tv (if it isn't already a smart TV) and you can beam any application that has the functionality to your TV and use your phone/tablet as the remote. Instead of loading it up on whatever your using pc/phone/tablet it goes direct to the Chromecast and to your TV cutting out the middle man.
You can pick one up for about $50 I think? Easy as hell to set up. As long as you've got a wifi connection you're good to go.
For now there are only limited apps but I'd imagine that will change very quickly. Youtube is obviously the big drawcard but you can use it as a wireless connection to your media centre via Plex and the like.