iTunes in the Cloud let’s you automatically download any App Store apps, iBookstore iBooks, and iTunes music you buy. For example, if you buy a game on your iPhone, it can automatically download onto your iPad. If you buy a song on iTunes on your Mac or Windows PC, it can automatically download onto your iPod touch.

via How to set up iTunes in the Cloud automatic downloads for apps, iBooks, movies, TV shows, and more | iMore.