Best described by this short quote:
Windows Phone 7 is the "generic" smartphone platform to target a broad audience, Kin is for a specific subset of super-connected kids.I think Microsoft gets it. The Facebook, MySpace, Twitter generation. They may be coming in later than Apple and Google, but their new stuff is very compelling. And the target audience is going to eat this stuff up. I bet the data plans will be cheaper too, as in less than the current $30 a month. We'll see when Verizon makes an announcement.
As for KIN itself, you can watch the UI videos on Engadget explaining how it works. If you only watch one, watch the fourth one on photo sharing. All photos are automatically backed up to the cloud - so now you don't have to worry about losing the phone or how much memory is on the phone (well not as much anyway). A great concept - I love it.
I believe KIN, along with Microsoft's Windows Phone 7, will bring Microsoft's phone business back from the brink.