Yapp is currently only developed for iOS and Android devices. This includes iPhones, iPod Touch devices, iPads, Android phones and Android tablets.
There has been little demand for Yapp for BlackBerry and Windows Phone and declining. Here is one useful link where we are keeping track of mobile OS market share.
For your users who don't have an iOS or Android device, they can still see the content of your app via the web on your app's web preview page.