In a Nutshell: Use your iPhone, iPad, Android phone, Android tablet and the like to "stream" YouTube, Netflix and other services (to be named later) to your HDTV. I put "stream" in quotes because that not how it really works. The streaming is straight from the Internet to the ChromeCast which is plugged in to an HDMI port on your HDTV.
What do you get out of that? A nice remote that's also your phone. It has a keyboard and mouse called your finger. Much better than using the clumsy interfaces that come with the normal HDTV Apps for these functions.
This is a very big deal. Partly because of the $35.
Thanks for the info.
I use a software application called PLEX to stream Internet content via my PC through the ROKU box to my TV.