What Is the difference between Google TV and Android TV set top boxes?

I've never used a Google TV, so is it only compatible with Google TV apps, and not the apps available for tablets/phones from the actual play store?

I know Android TV set top boxes is compatible with all google play store apps (at least I think), but the reason I'm asking this is because VIZIO has a sick Google TV releasing for $99, it does EVERYTHING, and way better & cheaper than Android TV set top boxes, but if the play store doesn't work for it, and only Google TV apps work i'll have to settle for Android TV.

Any information would be appreciated, thanks!