It's a device ($35) you plug into the HDMI slot of your tv and anything you're looking at in Google Chrome, you can view on your tv.
It's supposed to work with just things that are in Google Chrome.
But if you download the Videostream extension from the Chrome Store, you can also view any videos through the Chromecast device, too.
So for example, I have all 5 seasons of The Wire on my desktop. By clicking the Videostream app on my desktop, it opens up a Google Chrome webpage, and I find the movie to "upload" to the page and it streams to my tv.
There are other apps for Chromecast though. Netflix, Hulu Plus, WatchESPN...etc...