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.
An iTunes account can only have 10 devices enabled for iTunes in the Cloud at the same time. Devices include iPhones, iPod touches, iPads, as well as Mac and Windows PCs, and Apple TVs. If you reach 10, you can't add an additional devices until you remove one first. (See below.)
If you're not concerned about your cellular data cap or any potential overage charges, you can choose to have iTunes in the Cloud automatic downloads happen even when you're off Wi-Fi and on 3G or 4G.
If you do choose to allow 3G/4G downloads, there's still a maximum file size limit of 50MB. Any content bigger than that can only be downloaded over Wi-Fi.
If you still need help with re-downloading apps, games, iBooks, music, TV shows, or music to your iPhone, iPod touch, or iPad, or any other iCloud feature, head on over to our iCloud Forum and ask away, or check out our Ultimate guide to iCloud for everything you need to know!