Well, Forza Horizon is supposed to be a spin-off from the Forza games. Like, let's say, Sonic series has its spinoffs, Forza will have one too. It will be developed by a team of jolly people at PlayGround Games, and, as people say, it's an arcade game, sadly. No, not that kind of "Arcade" games like SEGA Rally or DAYTONA USA, but a game (well, much better than, but of the same kind as) like Test Drive.
It will be a little more N4Speedy, but, with the Forza physics, this game is going to be EPIC!
-quoted from the horizon forums.