You also need to connect power to it, either from a power outlet or from a USB-port on the TV. That's because the HDMI-port can't feed enough power to it.
Chromecast's wow-factor is that it can show the same content on the TV-screen as what you have on your iPad or iPhone. That doesn't mean you have to keep the iPad's screen on while watching. In fact you can press the home button or even turn the iPad off. I'm saying this so you get a feeling for how it works. You might say it turns your iPad/iPhone into a remote control. Needless to say I hope, Chromecast being Google's baby, you can use Android pads and phones too.
I use it to watch YouTube. With Chromecast you have exactly the same user interface as you have when watching YouTube videos on your iPad, including pause/play! That's the key feature of the device. A TV that offers a YouTube-button comes nowhere near the user friendliness of Chromecast.
You need a Chromecast enabled app running on the iPad/iPhone to use Chromecast. Video services and TV-companies all over the world are working hard to become Chromecast enabled. Netflix already has an app and so have these services HBO Go, Hulu Plus, Pandora, Google Play TV & Movies, Google Play Music, Avia, Plex, and many more.
The YouTube app enables you to create a play queue. All you have to do is decide whether you want to play a clip immediately or later. In the first case you press the play button, else you press the queue button.
If there's more than one iPad connected to the Chromecast you can take turns in selecting clips, but you can also enjoy fighting over who is the Chromecast master :-) We tried it here and had a great time.